emacs: cleanup and simplify `notmuch-show-archive-thread' and related functions

Recent changes in notmuch-show tagging introduced some code
duplication.  The patch cleanups and simplifies
`notmuch-show-archive-thread' function by using
`notmuch-show-tag-all', no longer used function are removed.  After
the change, `notmuch-show-archive-thread' function becomes symmetric
with `notmuch-show-archive-message'.

A side effect of these changes is that `notmuch-show-archive-thread'
no longer calls "notmuch tag" for each message in the thread.