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/testplatform.c

@@ -9,9 +9,6 @@
   including commercial applications, and to alter it and redistribute it
   freely.
 */
-
-#include <stdio.h>
-
 #include "SDL.h"
 
 /*
@@ -467,3 +464,5 @@ main(int argc, char *argv[])
 
     return status;
 }
+
+/* vi: set ts=4 sw=4 expandtab: */