Automatic merge from submit-queue (batch tested with PRs 55050, 53464, 54936, 55028, 54928). 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>.

kubelet: dockershim: remove orphaned checkpoint files

Fixes https://github.com/kubernetes/kubernetes/issues/55070

Currently, `ListPodSandbox()` returns a combined list of sandboxes populated from both the runtime and the dockershim checkpoint files.  However the sandboxes in the checkpoint files might not exist anymore.

The kubelet sees the sandbox returned by `ListPodSandbox()` and determines it shouldn't be running and calls `StopPodSandbox()` on it.  This generates an error when `StopContainer()` is called as the container does not exist.  However the checkpoint file is not cleaned up.  This leads to subsequent calls to `StopPodSandbox()` that fail in the same way each time.

This PR removes the checkpoint file if StopContainer fails due to container not found.

The only other place `RemoveCheckpoint()` is called, except if it is corrupt, is from `RemoveSandbox()`.  If the container does not exist, what `RemoveSandbox()` would have done has been effectively been done already.  So this is just clean up.

@derekwaynecarr @eparis @freehan @dcbw