tweener: Remove the onAnimationStart/onAnimationComplete callbacks

Our gnome-shell tweener integration has had hooks to determine when
the tweens have started and completed... except that they had a bug
in them. When a tween completed, it queued an idle handler to run
the callback in. If no tweens were running when the idle was removing,
it reset the tween state that contained the idle handler ID. It also
returned false, meaning that the source would always get removed.

If the actor had a tween in-flight when the idle was fired, it wouldn't
clean up after itself. While this is also a simple bug fix, remove the
callback so we don't queue unnecessary, unused idles.

https://bugzilla.gnome.org/show_bug.cgi?id=711732
dash: Make sure we clear the timeout IDs for the label tooltip

https://bugzilla.gnome.org/show_bug.cgi?id=711732