clutter/actor: Remove unused private function

`_clutter_actor_get_transition` is almost identical to
`clutter_actor_get_transition` and isn't used anywhere in the code,
remove it!

GNOME/mutter!791
5 jobs for !791 with small-clutter-actor-fixes in 6 minutes and 27 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Review
passed #428310
check-commit-log

00:00:21

 
  Build
passed #428311
build-mutter

00:02:52

passed #428312
build-without-native-backend

00:02:57

 
  Test
passed #428314
can-build-gnome-shell

00:02:24

passed #428313
test-mutter

00:03:08