Remove sampling from libcontainer CPU stats.

Sampling should be done by higher layer in configurable intervals. This
change removes the percentage stat as sampling is required for this.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge pull request #174 from vmarmol/no-sleep

Remove sampling from libcontainer CPU stats.