tst_QWidget::grabMouse() remove QApplicationPrivate::setActiveWindow()

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

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: I7fd9005ef66c11f640a50f0db468cdcb07eb621f
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

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

@@ -12099,7 +12099,6 @@ void tst_QWidget::grabMouse()
     layout->addWidget(grabber);
     centerOnScreen(&w);
     w.show();
-    QApplicationPrivate::setActiveWindow(&w);
     QVERIFY(QTest::qWaitForWindowActive(&w));
 
     QStringList expectedLog;