emacs: notmuch-tree-get-match: No longer define as command

When called from code, then this function returns non-nil when the
message at point is a matched message.  However it does nothing at all
to present that information to the user when it called interactively.
It is therefore safe to conclude that nobody is using this as a
command.

emacs/notmuch-tree.el

@@ -421,7 +421,6 @@ Some useful entries are:
 
 (defun notmuch-tree-get-match ()
   "Return whether the current message is a match."
-  (interactive)
   (notmuch-tree-get-prop :match))
 
 ;;; Update display