Remove the widget from the stylesheet cache before polishing

If the widget exists in the style rules cache before it polishes for the
first time then it should be removed from styleSheetCache too so that the
latest set stylesheet is used for the polishing.

Task-number: QTBUG-39427
Change-Id: Ic1e7988afe530f16ea9996bae56543ed554d6be9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>