message edit: Remove redundant if clause.

`topic_changed` must be true by this point, because
of the earlier return statement.

web/src/message_edit.js

@@ -869,10 +869,8 @@ export function save_inline_topic_edit($row) {
     show_topic_edit_spinner($row);
 
     if (message.locally_echoed) {
-        if (topic_changed) {
         message = echo.edit_locally(message, {new_topic});
         $row = message_lists.current.get_row(message_id);
-        }
         end_inline_topic_edit($row);
         return;
     }