tst_QMdiArea::currentSubWindow() remove setActiveWindow()

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

Remove redundant calls.

Task-number: QTBUG-121488
Change-Id: Idfdc6354c0083fff0b4d0d14213c08ac4415c950
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

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

@@ -1020,7 +1020,6 @@ void tst_QMdiArea::currentSubWindow()
 
     // Move focus to another top-level and check that we still
     // have an active window.
-    QApplicationPrivate::setActiveWindow(&dummyTopLevel);
     QCOMPARE(qApp->activeWindow(), (QWidget *)&dummyTopLevel);
     QVERIFY(mdiArea.activeSubWindow());