tst_QMdiArea::tabBetweenSubWindows() remove setActiveWindow()

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

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: I0c5f0e2fcc93c5ec8d15fafe9142cf948459a320
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

tests/auto/widgets/widgets/qmdiarea/tst_qmdiarea.cpp

@@ -2256,7 +2256,6 @@ void tst_QMdiArea::tabBetweenSubWindows()
     mdiArea.show();
     QVERIFY(QTest::qWaitForWindowExposed(&mdiArea));
 
-    QApplicationPrivate::setActiveWindow(&mdiArea);
     QWidget *focusWidget = subWindows.back()->widget();
     QCOMPARE(qApp->focusWidget(), focusWidget);