Remove unused white rectangle causing drawing issues

The spinbox has a white rectangle which overlaps the frames with some
styles (eg. oxygen). However the Rectangle itself serves no real purpose
and so it's removed.

Change-Id: I37a6862dfd8ac21fb24b54261fa7d7ab54aa23c7
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>