Do not return error from xcgroup_delete if cgroup already gone

Some cgroup code would retry continuously if somehow a cgroup
didn't exist when xcgroup_delete() was called (See for instance
proctrack/cgroup's slurm_container_plugin_wait()). To fix this
for all callers, return SUCCESS from xcgroup_delete() if rmdir(2)
return ENOENT, since the cgroup is already deleted.