To avoid an infinite loop, gimp_item_linked_is_locked() was temporarily
unlinking items before calling gimp_item_is_position_locked(). This
worked only based on gimp_item_real_is_position_locked() code which
called gimp_item_linked_is_locked() only when it was linked. It did not
take into account the fact that it was an abstract method which could
have other implementations. In particular the group layer implementation
would call in turn gimp_item_is_position_locked() on each child layer.

Basically temporarily unsetting the link was anyway a very ugly hack.
The point is simply that we are only interested by the value of the
`lock_position` flag for this item, without further "intelligence". For
this, use gimp_item_get_lock_position() instead.