QStyleSheetStyle: remove empty QRenderRule dtor

Prevents the compiler from synthesizing move assignment
operators for QRenderRule, thus ButtonInfo.

Change-Id: I3a4d67198606a7b84eeb9f454f4e076deb70d048
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>