[MessageTray] remove Source IDs

The tray itself does not actually need them, and to make status icon
sources work correctly the NotificationDaemon will need to be tracking
its sources by two separate IDs, so the existing system won't work.

Also remove MessageTray.removeSourceByApp(), which is
NotificationDaemon-specific, and implement the functionality in
notificationDaemon.js instead.

https://bugzilla.gnome.org/show_bug.cgi?id=627303

js/ui/telepathyClient.js

@@ -445,7 +445,7 @@ Source.prototype = {
     __proto__:  MessageTray.Source.prototype,
 
     _init: function(accountPath, connPath, channelPath, targetHandle, targetHandleType, targetId) {
-        MessageTray.Source.prototype._init.call(this, targetId, targetId);
+        MessageTray.Source.prototype._init.call(this, targetId);
 
         this._accountPath = accountPath;