tst_QWidget::grabKeyboard remove QApplicationPrivate::setActiveWindow()

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

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: Iea870711410dee1347f1020a6f7afc037e520825
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

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

@@ -12135,7 +12135,6 @@ void tst_QWidget::grabKeyboard()
     layout->addWidget(nonGrabber);
     centerOnScreen(&w);
     w.show();
-    QApplicationPrivate::setActiveWindow(&w);
     QVERIFY(QTest::qWaitForWindowActive(&w));
     nonGrabber->setFocus();
     grabber->grabKeyboard();