Remove invalid assert from QGraphicsScene

This assert has become invalid since commit 40fef403. After that commit, the
'origin' mentioned by the assert is not necessarily the first item of
cachedItemsUnderMouse, since it origin can be reassigned at line 5735.

Change-Id: Ibc0739b9fe31be3cc74110225940e9e173f585e0
Reviewed-by: Bernd Weimer <bweimer@rim.com>
Reviewed-by: Denis Dzyubenko <denis@ddenis.info>
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>

src/widgets/graphicsview/qgraphicsscene.cpp

@@ -5841,7 +5841,6 @@ bool QGraphicsScenePrivate::sendTouchBeginEvent(QGraphicsItem *origin, QTouchEve
                                                 firstTouchPoint.scenePos(),
                                                 static_cast<QWidget *>(touchEvent->target()));
     }
-    Q_ASSERT(cachedItemsUnderMouse.first() == origin);
 
     // Set focus on the topmost enabled item that can take focus.
     bool setFocus = false;