While code deleting old (relocated to new ones) IDs would work fine in
typical cases, it would fail badly in others, when e.g. drivers would
create 'reversed' dependency from the obdata ID to the object ID.
This commit uses a less efficient, but much safer method. It also
ensures no relocated old IDs is left over in the file (previous version
could easily leave some old IDs from the old library until a full
save/reload cycle happened).
