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/testdisplayinfo.c

@@ -12,11 +12,10 @@
 
 /* Program to test querying of display info */
 
-#include "SDL.h"
-
-#include <stdio.h>
 #include <stdlib.h>
 
+#include "SDL.h"
+
 static void
 print_mode(const char *prefix, const SDL_DisplayMode *mode)
 {
@@ -93,4 +92,3 @@ main(int argc, char *argv[])
 }
 
 /* vi: set ts=4 sw=4 expandtab: */
-