QMacStyle: Remove cached focus ring drawing

We go for direct drawing for now. Some of the logic was
a bit sketchy, and we need to measure how useful is caching
and eventually compatible with the focus frame animation.

Change-Id: Id30a68f77a4129197536e8ca251906ee8a730925
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>