QWindowsVistaStyle::drawPrimitive(PE_Frame): fix background clipping

The purpose of PE_Frame is to draw the frame, not the background. The
old code tried to check whether a custom base brush was set, and
otherwise let the theme draw the background. That didn't work together
with CSS background images. This change removes the background drawing
from PE_Frame, and let's PE_Widget to draw the background using the
base brush that defaults to a color fetched from the theme.

Task-number: QTBUG-24783
(cherry picked from qtbase/f1e681bed21e131864fe1e1c91679f7a56b06823)

Change-Id: Ief256d1d5861abc95dedf0b5d0a4f8e338c48941
Reviewed-by: Andy Shaw <andy.shaw@digia.com>