Remove HAVE_CONFIG_H defs and uses

Since GLib files are only meant to be built as part of GLib, config.h
always exists, so the checks are more or less pointless.

https://bugzilla.gnome.org/show_bug.cgi?id=793399

gio/xdgmime/xdgmimemagic.c

@@ -23,9 +23,7 @@
  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifdef HAVE_CONFIG_H
 #include "config.h"
-#endif
 
 #include <assert.h>
 #include "xdgmimemagic.h"