notmuch.el: Don't try to remove an "unread" tag that's not there.

This optimization wouldn't be necessary if we had a nice fast "notmuch
tag" command. But since it's currently fairly slow, (see Xapian defect
250: http://trac.xapian.org/ticket/250), we're willing to take some
extra care to avoid calling "notmuch tag" unnecessarily.