If llStopMoveToTarget() is called on an attachment, then stop the avatar if it was moving to target.

Same behaviour as on Linden Lab grid.
Will probably also address http://opensimulator.org/mantis/view.php?id=7369 by not generating spurious object updates when llStopMoveToTarget() is called in attachments where the avatar is not moving.
Remove SOP.StopMoveToTarget scheduled update which is now being done in SOG.stopMoveToTarget() as of last commit 67e568