TelepathyClient: don't emit an error if an unknown message is acknowledged

pending-messages-removed is emitted for sent messages too, but we don't
include those in the _pendingMessages list. Avoid useless spew in the session
logs in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=683449

js/ui/components/telepathyClient.js

@@ -746,8 +746,6 @@ const ChatSource = new Lang.Class({
             this._pendingMessages.splice(idx, 1);
             this.countUpdated();
         }
-        else
-            throw new Error('Message not in our pending list: ' + message);
     },
 
     _ackMessages: function() {