Don't use kill-this-buffer to kill notmuch emacs buffers

Don't use kill-this-buffer to kill notmuch emacs buffers

kill-this-buffer appears to be a function intended specifically for
use in the menu bar, and causes problem killing notmuch buffers when
multiple frames have been used.  This patch replaces kill-this-buffer
with notmuch-kill-this-buffer, which in turn just simply calls
(kill-buffer (current-buffer)).