Style: Remove useless background paint for menu scroller

This would also cause an extra paint in QQuickStyleItem, resulting
in the frame being erased for those styles not having a frame width
set (e.g., fusion and GTK).

Change-Id: I7a9371c540cd31fd9f1400a51c4ec57582996dd2
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/widgets/styles/qcommonstyle.cpp

@@ -1329,7 +1329,6 @@ void QCommonStyle::drawControl(ControlElement element, const QStyleOption *opt,
         break;
 #ifndef QT_NO_MENU
     case CE_MenuScroller: {
-        p->fillRect(opt->rect, opt->palette.background());
         QStyleOption arrowOpt = *opt;
         arrowOpt.state |= State_Enabled;
         proxy()->drawPrimitive(((opt->state & State_DownArrow) ? PE_IndicatorArrowDown : PE_IndicatorArrowUp),