proctrack/cgroup: no longer rely on release agent to clean step cg

With release_agent notified at the step cgroup level, the step cgroup
can be removed while slurmstepd as not yet finished its internals
epilog mechanisms. Inhibiting release agent at the step level and
ensuring its proper removal helps to guarantee that the node will only
be eligible for job execution when the resources will be completely
available (no longer used by the job or the epilogs).