cleanup: Use arrow functions for tweener callbacks

While it is legal to use method syntax for the function properties
here, arrow notation is less unexpected and allows us to drop the
separate scope properties.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/608

js/ui/messageList.js

@@ -469,8 +469,7 @@ var Message = class Message {
                              { scale_y: 0,
                                time: MessageTray.ANIMATION_TIME,
                                transition: 'easeOutQuad',
-                               onCompleteScope: this,
-                               onComplete() {
+                               onComplete: () => {
                                    this._actionBin.hide();
                                    this.expanded = false;
                                } });