Remove checks for outdated GCC versions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/cpp.h

@@ -140,20 +140,6 @@
 
 #ifdef HAVE_VARIADIC_MACROS
 
-/*
-   This is a hack to make it possible to use variadic macros with g++ 3.x even
-   when using -pedantic[-errors] option: without this, it would complain that
-
-       "anonymous variadic macros were introduced in C99"
-
-   and the option disabling this warning (-Wno-variadic-macros) is only
-   available in gcc 4.0 and later, so until then this hack is the only thing we
-   can do.
- */
-#if defined(__GNUC__) && __GNUC__ == 3
-    #pragma GCC system_header
-#endif /* gcc-3.x */
-
 /*
    wxCALL_FOR_EACH(what, ...) calls the macro from its first argument, what(pos, x),
    for every remaining argument 'x', with 'pos' being its 1-based index in