Bug 535647 - Deleting a reference property allocated to a resource,
wrongly deletes the property itself

Added special logic so that a reference property is deleted in the same
way as a literal property. namely, by deleting the allocation, and not
the property itself.
Default behaviour in Sirius is to delete the property element, which is
not the desired effect here.

Change-Id: I71c81baee9f13d51f1b9dd5157707781820396e7
Signed-off-by: Jad El-khoury <jad@kth.se>