dnd: Fix syntax error which led into g-s not starting anymore

With the recent port to JS6 classes, the trailing
comma after functions in the syntax of classes has
been removed.
However commit c2961f21 accidentally reintroduces
one trailing comma after a newly created function,
leading into g-s throwing an exception and not
starting anymore.

Therefore, remove this trailing comma to solve
this problem.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/366

js/ui/dnd.js

@@ -226,7 +226,7 @@ var _Draggable = class _Draggable {
         }
 
         return false;
-    },
+    }
 
     _onEvent(actor, event) {
         let device = event.get_device();