messageTray: Remove support for Music notifications

Music is no longer a special type of notification according
to the design. If we want to resurrect the functionality, we
can reimplement it with a dedicated API like MPRIS rather
than piggy-bagging on the notification system.

https://bugzilla.gnome.org/show_bug.cgi?id=744815

js/ui/notificationDaemon.js

@@ -351,9 +351,6 @@ const FdoNotificationDaemon = new Lang.Class({
                 }));
         }
 
-        // Mark music notifications so they can be shown in the screen shield
-        notification.isMusic = (ndata.hints['category'] == 'x-gnome.music');
-
         let gicon = this._iconForNotificationData(icon, hints);
         let gimage = this._imageForNotificationData(hints);