Revert "Reset QWidget's winId when backing window surface is destroyed"

This reverts commit a9246c7132.

The QWidget machinery is way to fragile to reset the winId under the
feet of QWidget like that. We would potentially need to include all
the logic in QWidget::destroy. This also ties into the flow between
QtGui and QtWidgets during window closing, which is still unresolved.

Change-Id: I168048a63c89796398eb5331a80ce3e5c8d9a208
Fixes: QTBUG-76588
Task-number: QTBUG-69289
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -9382,12 +9382,6 @@ bool QWidget::event(QEvent *event)
         d->renderToTextureReallyDirty = 1;
 #endif
         break;
-    case QEvent::PlatformSurface: {
-        auto surfaceEvent = static_cast<QPlatformSurfaceEvent*>(event);
-        if (surfaceEvent->surfaceEventType() == QPlatformSurfaceEvent::SurfaceAboutToBeDestroyed)
-            d->setWinId(0);
-        break;
-    }
 #ifndef QT_NO_PROPERTIES
     case QEvent::DynamicPropertyChange: {
         const QByteArray &propName = static_cast<QDynamicPropertyChangeEvent *>(event)->propertyName();