Mistake in own rB358155a8da60, change ended up discarding the case where
we need to also delete IDs using a tagged-to-be-deleted ID in a 'never
NULL' way. Typical example: Whene deleting a Mesh ID, one also needs to
delete all the Objects using that mesh, since obdata pointer is of type
'never NULL'.
Note that luckily, this fix does not affect the performance improvements
from rB358155a8da60.
Noted by Brecht and Clement because of failing unittests, shame on me.
ceb0e7fcea