Don't check window system when creating SceneQPainter

Summary:
For one SceneQPainter is only used on Wayland and it can be created
through the Platform, so no different code pathes needed even if we
had SceneQPainter on X11.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1416