[PanelView] remove outdated hack to support mask without compositing

Summary:
The actual mask is now queried from the panel, there is no more need
for this.

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: davidedmundson, mvourlakos, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21013