tst_QWidget::showAndMoveChild() remove setActiveWindow()

2f6fe3a26843ff68c5d3f9af0a2fc3cce6caac22 has made calls to
QApplicationPrivate::setActiveWindow() redundant.

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: I7bd841574c07d73416f9f63d564fe31a8475fa9e
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -6813,7 +6813,6 @@ void tst_QWidget::showAndMoveChild()
     parent.setGeometry(desktopDimensions);
     parent.setPalette(Qt::red);
     parent.show();
-    QApplicationPrivate::setActiveWindow(&parent);
     QVERIFY(QTest::qWaitForWindowActive(&parent));
 
     QWidget child(&parent);