taskQTBUG_42915_focusNextPrevChild() remove setActiveWindow()

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

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: I8f55afeb893937029e3ba05f141b9e4494ecd2d5
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

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

@@ -4952,7 +4952,6 @@ void tst_QGraphicsScene::taskQTBUG_42915_focusNextPrevChild()
     widget2->setFocusPolicy(Qt::NoFocus);
 
     view.show();
-    QApplicationPrivate::setActiveWindow(&view);
     QVERIFY(QTest::qWaitForWindowActive(&view));
 
     QTest::keyEvent(QTest::Click, &view, Qt::Key_Tab);