clutter/actor: Revert "Use different view list when picking frame clock of stage"
This reverts commit 9693462f.
Because returning NULL is already handled properly by all callers of
clutter_actor_pick_frame_clock
. In particular, clutter-timeline's
update_frame_clock
will defer and reattempt the call after
stage-views-changed
has been signalled.
Fixes: gnome-shell#3571 (closed)