Remove one of two uses of Dockershim in `cmd/kubelet`. The other is for
creating a docker client which we pass to the Kubelet... we will handle
that refactor in a separate diff.
I'm fairly confident, though need to double check, that no one is
actually using this experimental dockershim behavior. If they are, I
think we will want to find a new way to support it (that doesn't require
using the Kubelet only to launch Dockershim).