ci-buildpackage: Drop debian/apertis/ cleanup hook

We can now drop the debian/apertis/ cleanup hook since:

1. we no longer use debian/apertis/gitlab-ci.yml since we have switched to an external reference
2. debian/apertis/ won't become empty since we always have at least a component file

This also makes ci-buildpackage work with non-Apertis packages which were previously failing on the rmdir invocation no handling well the lack of the debian/apertis/ folder.