Restore previous client AO behaviour by not allowing them to remove the default animation but continue to allow scripts to do so.

This keeps the fix from http://opensimulator.org/mantis/view.php?id=6327
and fixes the behaviour regression in http://opensimulator.org/mantis/view.php?id=6483
Animations may still exhibit different behaviour if both scripts and clients are adjusting animations.
A change in the behaviour of client AO to not remove all animations may be a better long term approach.