SDL_internal: remove #undef SDL_LEAN_AND_MEAN

I am confused by the "You have to manually edit this file" comment. Does it mean that it is expected to manually remove the previous `#define SDL_LEAN_AND_MEAN 0` ?
In any case I put this part of the change in a separate commit so that it can easily be reverted.

src/SDL_internal.h

@@ -127,9 +127,7 @@
 #endif
 
 /* A few #defines to reduce SDL footprint.
-   Only effective when library is statically linked.
-   You have to manually edit this file. */
-#undef SDL_LEAN_AND_MEAN
+   Only effective when library is statically linked. */
 
 /* Optimized functions from 'SDL_blit_0.c'
    - blit with source bits_per_pixel < 8, palette */