Don't count a message as unseen if it is deleted -- even if the SEEN

Don't count a message as unseen if it is deleted -- even if the SEEN
flag isn't set.  It's common to delete messages without seeing them,
and the act of deletion is an indication that the user isn't
interested in seeing them.

However, don't mark these messages as seen; they might later be
undeleted, in which case they want to retain their unseen status.