Remove checks for outdated GCC versions

wxWidgets now supports only GCC 4 and newer, so there is no reason
to keep code specific for older GCC versions.

include/wx/testing.h

@@ -503,11 +503,6 @@ private:
  */
 #ifdef HAVE_VARIADIC_MACROS
 
-// See wx/cpp.h for the explanations of this hack.
-#if defined(__GNUC__) && __GNUC__ == 3
-    #pragma GCC system_header
-#endif /* gcc-3.x */
-
 #define wxTEST_DIALOG(codeToRun, ...)                                          \
     {                                                                          \
         wxTEST_DIALOG_HOOK_CLASS wx_hook(__FILE__, __LINE__, __WXFUNCTION__);  \