Use cgroup paths for stats and removal

This builds on the commit in #266 and implements removal of the cgroups via these paths and a simple `os.RemoveAll`.

This also removes systemd's version of getstats because we already have the raw paths that were setup during apply.  This allows the normal fs code to collect stats at these paths.  

This should also fix the issues in: #200

Fixes #198
Closes #200 
Closes #266
nit: Maybe have this function in cgroup so it gets shared throughout?