Revert "Add a QA check for systemd unit file install."

This reverts commit dd2d661078771a41d4fd554c2bc3b1188ce7b53e.
Forcing use of a specific eclass is too restrictive. It would be better
to use a variable to control a warning like this. See bug #465562.
depgraph: don't _skip_restart for blockers

This prevents blockers from interfering with backtracking, as reported
in bug #465356, comment #15.