backend: Remove unused barrier vfunc

It was added in error, and has been unused since its introduction. Lets
correct the error.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2604>

src/backends/meta-backend.c

@@ -1589,14 +1589,6 @@ meta_backend_set_client_pointer_constraint (MetaBackend           *backend,
   g_set_object (&priv->client_pointer_constraint, constraint);
 }
 
-MetaBarrierImpl *
-meta_backend_create_barrier_impl (MetaBackend *backend,
-                                  MetaBarrier *barrier)
-{
-  return META_BACKEND_GET_CLASS (backend)->create_barrier_impl (backend,
-                                                                barrier);
-}
-
 ClutterBackend *
 meta_backend_get_clutter_backend (MetaBackend *backend)
 {