notmuch show: Remove custom "unread" hack, (printing tag in two locations).

I previously had a hack that special-cased the "unread" tag and
printed it on the same line as the message ID. But now that we are
printing all tags at the end of the one-line summary we don't need
this anymore. Get rid of it, and just read "unread" from the list of
tags just like any other tag.