Drop xcb-shm usage from QPainterWindowPixmap

Summary:
Before we were able to render Xwayland windows through the Wayland buffer
we used a xcb-shm to map the window data in the QPainter compositor.

As we don't use that any more and QPainter is not available for X11
anyway we can just drop the code.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1193