Remove SDL_config.h from the public headers

The SDL headers are no longer dependent on the build configuration.

Fixes https://github.com/libsdl-org/SDL/issues/6643 and https://github.com/libsdl-org/SDL/issues/6641

test/testnative.c

@@ -11,7 +11,6 @@
 */
 /* Simple program:  Create a native window and attach an SDL renderer */
 
-#include <stdio.h>
 #include <stdlib.h> /* for srand() */
 #include <time.h> /* for time() */