clutter: Remove CLUTTER_ACTOR_IN_REPARENT flag

The CLUTTER_ACTOR_IN_REPARENT and the CLUTTER_IN_REPARENT flag are never set and the logic for skipping unmap, unrealize and the emission of the "parent-set" signal during reparents has been solved differently by leaving out the CHECK_STATE and EMIT_PARENT_SET flags when calling add_child_internal() and remove_child_internal().

The only place where those REPARENT flags are theoretically still useful is in the clutter_actor_verify_map_state() debugging function, but that is never called during reparent anyway, so simply leave the comment regarding reparent there.