vlc_object_attach: do not kill an object within the structure lock

vlc_object_attach: do not kill an object within the structure lock

Killing a the parent within vlc_object_attach looks like an awful hack
anyway. And there was a documented race condition, in addition to the
dead lock.

This reverts all the pluses from 99e8bcdd. vlc_object_kill() takes the
structure lock internally (through vlc_list_child()), so this would
always deadlock. Besides, we cannot take any lock within the structure
lock, lest we forbid any thread from ever manipulating the objects
structure while holding a lock on any object.
AFAIK, only object reference spins and the messages bank can be locked
within the structure lock.