forceRendering was introduced for QQuickWidget, but QQuickWidget
now has full control of when the rendering functions are called. It
makes more sense to not call the QQuickRenderControl functions, than to
set a flag on QQuickWidget that decides whether those functions have any
effect.

Change-Id: I69819b13000120c04cecd1a467c08e8df9330df8
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>