tast187791_setSceneCausesUpdate() remove setActiveWindow()

2f6fe3a268 has made calls to
QApplicationPrivate::setActiveWindow() redundant.

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: Iaaff78826e74df119d4c4a8ea376e1b414da3c3d
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

tests/auto/widgets/graphicsview/qgraphicsview/tst_qgraphicsview.cpp

@@ -3246,7 +3246,6 @@ void tst_QGraphicsView::task187791_setSceneCausesUpdate()
     QGraphicsScene scene(0, 0, 200, 200);
     QGraphicsView view(&scene);
     view.show();
-    QApplicationPrivate::setActiveWindow(&view);
     QVERIFY(QTest::qWaitForWindowExposed(&view));
 
     EventSpy updateSpy(view.viewport(), QEvent::Paint);