Drop the explicit fallback to XRender

Summary:
With the new try of all compositor types supported there is an automatic
fallback from OpenGL to XRender/QPainter in case OpenGL setup failed.
So there is no need to invoke a method to do just that.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8364