[platform/virtual] Drop support for vgem and rendernode

Summary:
Instead we depend on the surfaceless platform for which we recently
added support. Thus the plugin does not need to use gbm and udev
anymore. So simplifies a lot.

Test Plan: ctest (prior to breaking change) passes

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18160