main: Don't gdk_flush() after every main loop wait

For a start, this makes gtk_main() work different from g_main_loop_run()
calls.

But most importantly, modern GDK does proper syncing itself and doesn't
need to rely on a catch-all to get it right.

gtk/gtkmain.c

@@ -986,7 +986,6 @@ gtk_main (void)
       gdk_threads_leave ();
       g_main_loop_run (loop);
       gdk_threads_enter ();
-      gdk_flush ();
     }
 
   main_loops = g_slist_remove (main_loops, loop);