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/testtimer.c

@@ -13,10 +13,6 @@
 /* Test program to check the resolution of the SDL timer on the current
    platform
 */
-
-#include <stdlib.h>
-#include <stdio.h>
-
 #include "SDL.h"
 
 #define DEFAULT_RESOLUTION  1