Drop the LXC OSTree containers

In the past we started a proof-of-concept implementation of self-updating OSTree-based LXC containers, but in the end the project which prompted this development ended up updating containers from the host.

Since we have no planned use for them, they are not part of any formal release, no test is performed on them and since they fell quite behind after the rebase to Buster with several parts still commented out, let's drop them altogether.

Test build: https://jenkins.apertis.org/job/playground/job/em/job/apertis-v2019dev0/job/images/job/debos-image-build/17/