notmuch show: Don't show the subject line twice.

notmuch show: Don't show the subject line twice.

I recently added a print of the subject line for use as part of a
two-line summary in the emacs client. But of course, the subject was
already being printed on the next line. So I didn't really need to add
anything, I could have just stopped hiding what was already
printed. Anyway, we now avoid printing it twice in a row.