Revert "render: Set renderer->window to NULL in SDL_DestroyRendererWithoutFreeing."

This reverts commit 9f8dffbd2d.

This causes some tests to fail, and wasn't otherwise a necessary change, so
I'm backing it out.

(Looks like some sort of interaction with software renderers and their
surfaces not getting destroyed...?)

src/render/SDL_render.c

@@ -4603,7 +4603,6 @@ void SDL_DestroyRendererWithoutFreeing(SDL_Renderer *renderer)
 
     if (renderer->window) {
         SDL_ClearProperty(SDL_GetWindowProperties(renderer->window), SDL_PROP_WINDOW_RENDERER_POINTER);
-        renderer->window = NULL;
     }
 
     /* Free the target mutex */