clutter: Remove ability to externally set sync-to-vblank

Externally setting the sync-to-vblank setting was a feature
added as a workaround to old Intel and ATI graphic cards, and
is not needed anymore. Furthermore, it doesn't make sense to
change it on a compositor whatsoever.

This commit removes all the ways to externally change this
setting, as well as the now unused API.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/191

src/backends/native/meta-renderer-native.c

@@ -2729,9 +2729,6 @@ meta_renderer_native_create_onscreen (MetaRendererNative   *renderer_native,
 
   /* We have wait_for_pending_flips hardcoded, so throttling always. */
   cogl_onscreen_set_swap_throttled (onscreen, TRUE);
-  if (!_clutter_get_sync_to_vblank ())
-    g_warning ("Request to disable sync-to-vblank is being ignored. "
-               "MetaRendererNative does not support disabling it.");
 
   if (!cogl_framebuffer_allocate (COGL_FRAMEBUFFER (onscreen), error))
     {