minor: Avoid ui_util.focus_on() call.

The helper is unnecessary at this point and obscures
greps for selectors.  We completely eliminate it in
the next commit.

static/js/message_edit.js

@@ -529,7 +529,7 @@ exports.edit_last_sent_message = function () {
     // Finally do the real work!
     compose_actions.cancel();
     message_edit.start(msg_row, function () {
-        ui_util.focus_on('message_edit_content');
+        $('#message_edit_content').focus();
     });
 };