mu4e: update font-locking for mu4e-compose-mode

mu4e was making a vain attempt to fontify the compose buffer; this
doesn't work because message (from which mu4e-compose-mode derives) uses
font-locking for that.

So, instead, remap the message-mode faces to the ones used for mu4e.

mu4e/mu4e-draft.el

@@ -99,7 +99,6 @@ its settings apply."
 	(funcall mu4e-compose-cite-function))
       (pop-mark)
       (goto-char (point-min))
-      (mu4e~fontify-cited)
       (buffer-string))))
 
 (defun mu4e~draft-header (hdr val)