Often when the reference linked data is significantly modified, a lot of
'ghost' linked data remain referenced by liboverrides, even after
resync. This is due to the fact that missing data is ignored (skipped)
during resync process, to avoid potential destruction of data in case
the linked data is actually missing.
However, after all resync has been done, we can consider that missing
linked references and their liboverrides can be safely deleted, if the
later are not user-edited or hierarchy roots.
Bastien Montagne
bef276ab0b