Remove usage of private call QWidgetPrivate::updateWidgetTransform()

It used to be needed in 5.6 for IME window placement purposes, but
since 5.7 when we switched to using QQuickWidget instead of
QOpenGLWidget as a result of commit
800365f6

, the updateWidgetTransform
gets called implicitly because we forward FocusIn events to
QQuickWidget::event, which forwards them to QWidget::event which
calls updateWidgetTransform for us.

Task-number: QTBUG-63098
Change-Id: I0a0ba50c1491797b37765d26d761c358c156950f
Reviewed-by: Michael Brüning <michael.bruning@qt.io>