wayland: Remove direct session

Most distros use either systemd's logind or consolekit for session
management.

The main reason why there's direct session is because up to some point,
kwin had a class called VirtualTerminal which was kind of like a direct
session backend.

The VirtualTerminal was used in the fbdev backend in conjuction with the
logind session backend, which looked odd.

Since the drm backend supported only logind and consolekit for very
long time and we hadn't received any complaints about it, the fact that
direct session is unsupported should not be noticeable.

Given that, this change removes the support for direct session in order
to reduce the amount of platform-specific code and make it easier to
maintain kwin.