We need to completely remove a message from over.sqlite3 and
Xapian when no references remain, otherwise users will still see
the removed messages in NNTP overviews and WWW search
results/summaries.
References to messages are now solely handled by the `xref3'
table of over.sqlite3. We can also trust `xref3' when deciding
whether to remove only the "O$eidx_key" and "G$lid" terms from a
document in Xapian or to remove the entire Xapian document.