emacs: do not call `notmuch-hello-mode' on update

`notmuch-hello' should call `notmuch-hello-mode' function only when
run for the first time.  But before the change, `notmuch-hello' used
`kill-all-local-variables' to remove editable widgets fields.  This
caused the major mode to be reset, and `notmuch-hello-mode' to be
called every time.

The patch manually deletes all editable widget fields and removes
`kill-all-local-variables' call.