Automatic merge from submit-queue (batch tested with PRs 52240, 48145, 52220, 51698, 51777). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>..
dockershim: remove support for legacy containers
The code was first introduced in 1.6 to help pre-CRI-kubelet upgrade to
using the CRI implementation. They can safely be removed now.