Remove the duplicate pid check in Docker containerizer

In `DockerContainerizerProcess::_recover`, we check if there are two executors use duplicate pid, and error out if we find duplicate pid (see here for details). However I do not see the value this check can give us but it will cause serious issue (agent crash loop when restarting) in rare case (a new executor reuse pid of an old executor), so I think we'd better to remove it from Docker containerizer.