TECHNICAL FIELD The present disclosure relates to a method of recycling resources when manufacturing a lithium secondary battery. In particular, the present disclosure relates to a method of recovering and reusing a positive electrode active material from a positive electrode scrap generated in a lithium secondary battery manufacturing process or from a lithium secondary battery that is discarded after use. The present application claims priority to Korean Patent Application No. 10-2020-0082622 filed on July 06, 2020 in the Republic of Korea, the disclosure of which is incorporated herein by reference. BACKGROUND ART Lithium secondary batteries which may be repeatedly charged and discharged are in the spotlight as an alternative to fossil energy. Lithium secondary batteries have been mainly used in traditional handheld devices such as cell phones, video cameras, and power tools. However, recently, application fields of lithium secondary batteries have been gradually increasing to electric vehicles (EVs, HEVs, and PHEVs), large capacity energy storage systems (ESSs), uninterruptible power supply systems (UPS), etc. A lithium secondary battery includes an electrode assembly in which unit cells having a structure in which a positive electrode plate and a negative electrode plate coated with an active material on a current collector are arranged with a separator interposed PCT/KR2021/008205 2021-06-29 3 therebetween, and an exterior material sealing and accommodating the electrode assembly together with an electrolyte, that is, a battery case. The positive electrode active material of the lithium secondary battery mainly uses a lithium-based oxide, and the negative electrode active material uses a carbon material. The lithium-based oxide contains a metal such as cobalt, nickel, or manganese. In particular, cobalt, nickel, and manganese are very expensive valuable metals. Among these, cobalt is a strategic metal, and each country in the world has a special interest in supply and demand of cobalt. Since the number of cobalt producing countries is limited, it is known as a metal whose supply and demand is unstable worldwide. If an imbalance in the supply and demand of raw materials of strategic metals occurs, raw material prices are highly likely to rise. Conventionally, research on recovering and recycling these valuable metals from lithium secondary batteries (waste batteries) which are discarded when their lifespan is completed after use has been mainly conducted. In addition to waste batteries, it is more preferable if resources may be recovered from wastes discarded after the positive electrode plate is punched or from the positive electrode in which a defects occur during the process. Currently, when manufacturing a lithium secondary battery, as shown in FIG. 1, a positive electrode sheet 30 is manufactured by forming a positive electrode active material layer 20 in which a long sheet type positive electrode current collector 10 such as aluminum (Al) foil is coated with a positive electrode slurry in which a positive electrode active material, a conductive material, a binder, a solvent, etc. are mixed, and then a positive electrode plate 40 is punched to a certain size. A part remaining after punching is discarded as a positive electrode scrap 50. If it is possible to recover the positive electrode active material from the positive electrode scrap 50 and reuse the positive electrode active material, PCT/KR2021/008205 2021-06-29 4 it would be very desirable from an industrial-economic point of view and an environmental point of view. Conventionally, in most cases, a method of recovering the positive active material dissolves a positive electrode in hydrochloric acid, sulfuric acid, nitric acid, etc., and then extracting an active material element such as cobalt, nickel, manganese, etc., and reuses the active material element as a raw material for the synthesis of the positive active material. However, the method of extracting the active material element using an acid has disadvantages in that a process of or recovering pure raw materials is not environmentally friendly as well as requires a neutralization process and a wastewater treatment process, which increases the process cost. In addition, the method has a disadvantage in that lithium, which is one of the main elements of the positive electrode active material, may not be recovered. In order to solve these disadvantages, a method of directly reusing the active material without dissolving the positive electrode active material and extracting the active material in element form is required. Furthermore, a method of maximally preventing the loss of constituent elements such as lithium during the process of obtaining a reuse active material is desirable. The loss of constituent elements such as lithium must be prevented as much as possible so as not to be significantly different from the composition of a fresh active material that has never been used. By doing so, a process of adding insufficient components may be minimized. DISCLOSURE Technical Problem The present disclosure is designed to solve the problems of the related art, and PCT/KR2021/008205 2021-06-29 5 therefore the present disclosure is directed to providing a method of recovering and reusing an active material from a positive electrode scrap. Technical Solution In one aspect of the present disclosure, there is provided a positive electrode active material reuse method including (a) by thermally decomposing a binder and a conductive material in an active material layer by performing heat treatment in air on a positive electrode scrap including a lithium composite transition metal oxide positive electrode active material layer on a current collector, separating the current collector from the active material layer and recovering an active material in the active material layer, (b) washing the recovered active material with a cleaning solution, and (c) obtaining a reusable active material by adding a lithium precursor to the washed active material and annealing the active material, wherein a molar ratio of lithium to other metals in the active material after heat treatment or a molar ratio of lithium to other metals in the active material after washing has a decrease range within 20% compared to a molar ratio of lithium to other metals in the positive electrode scrap before heat treatment. In the present disclosure, the positive electrode active material reuse method may further include (d) performing surface coating on the annealed active material. Heat treatment may be performed at 300 to 650 °C, in particular, for 10 minutes to 24 hours. Heat treatment may be performed at 550 °C as a temperature increase rate of 5 °C/min for 30 minutes. The cleaning solution may be water. Alternatively, the cleaning solution may be a PCT/KR2021/008205 2021-06-29 6 lithium compound aqueous solution showing basicity in an aqueous solution state. The lithium compound aqueous solution may be prepared to contain a lithium compound more than 0% and equal to or less than 15%, and preferably uses LiOH. The washing may be performed within one week, preferably within one day, and more preferably within one hour. Preferably, the cleaning solution in which a dissolving amount of LiF is 0.127g/100ml (18 ℃) and 0.134g/100ml (25 ℃) is used, and a ratio of the active material to the cleaning solution during washing is equal to or less than 1:200 and preferably 1:30 or more. The washing may be performed by impregnating the recovered active material in the lithium compound aqueous solution and at the same time stirring the recovered active material. The lithium precursor used in annealing may include at least one of LiOH, Li2CO3, LiNO3 and Li2O. The lithium precursor is added by an amount that may be added as much as a ratio of lithium lost compared to a ratio of lithium to other metals in a raw material active material used in the active material layer. For example, the lithium precursor may be added by an amount of lithium added at a molar ratio of 0.001 to 0.4. Furthermore, the lithium precursor may be added by an amount of lithium that may be further added at a molar ratio of 0.0001 to 0.1 with respect to 1:1 that is a molar ratio of lithium to other metals. This additionally added lithium is used as a material of a surface protective layer in the performing of surface coating on the annealed active material. For another example, without drying after the washing, the lithium precursor may be added in step (c) by mixing the washed active material in a lithium precursor solution and PCT/KR2021/008205 2021-06-29 7 spray drying the active material. At this time, a temperature of the spray drying is preferably100 to 300 ℃. The annealing may be performed at 400 to 1000 ℃ in the air. A temperature of the annealing may exceed a melting point of the lithium precursor. The active material in the active material layer may be recovered in powder form, and a carbon component generated by carbonization of the binder or the conductive material may not remain on a surface. The performing of the surface coating may include coating at least one of a metal, an organic metal and a carbon component on a surface in a solid or liquid method and then performing heat treatment at 100 to 1200 ℃. The reusable active material may be represented by Chemical Formula 1 below, LiaNixMnyCozMwO2+δ (in Chemical Formula 1 above, M includes at least one selected from the group consisting of B, W, Al, Ti and Mg, 1