Removed the need for SDL_CreateTLS()

This eliminates the tap dancing needed for allocating TLS slots, we'll automatically allocate them as needed, in a thread-safe way.

build-scripts/SDL_migration.cocci

@@ -2630,11 +2630,6 @@ typedef SDL_cond, SDL_Condition;
 + SDL_WINDOW_HIGH_PIXEL_DENSITY
 @@
 @@
-- SDL_TLSCreate
-+ SDL_CreateTLS
-  (...)
-@@
-@@
 - SDL_TLSGet
 + SDL_GetTLS
   (...)