tst_QGraphicsScene::tabFocus_emptyScene() remove setActiveWindow()

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

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: I5608d71782b47f6aa0cfbdbb077980779052bc41
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

tests/auto/widgets/graphicsview/qgraphicsscene/tst_qgraphicsscene.cpp

@@ -3044,7 +3044,6 @@ void tst_QGraphicsScene::tabFocus_emptyScene()
     widget.setLayout(layout);
     widget.setWindowTitle(QTest::currentTestFunction());
     widget.show();
-    QApplicationPrivate::setActiveWindow(&widget);
     widget.activateWindow();
     QVERIFY(QTest::qWaitForWindowActive(&widget));