Remove Qt 5 to-do comment that will not be addressed.

The suggested behavioural change had the potential to break existing
code, so the change won't be made for Qt 5.

Task-number: QTBUG-25119
Change-Id: Ie03271d12b21a800c998e073eeb9ca1cd03ffe19
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/widgets/kernel/qwidget.cpp

@@ -6730,7 +6730,6 @@ void QWidget::setContentsMargins(int left, int top, int right, int bottom)
     else
         updateGeometry();
 
-    // ### Qt 5: compat, remove
     if (isVisible()) {
         update();
         QResizeEvent e(data->crect.size(), data->crect.size());