Automatic merge from submit-queue (batch tested with PRs 55511, 63372, 63400, 63100, 63769). 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>.

k8s.io/client-go/{kubernetes,rest}: remove dependency on pflag

This change removes an unused pflag utility which cuts a client-go transitive dependency on that library.

Verified using `go list`

```
go list -f '{{join .Deps "\n"}}' k8s.io/kubernetes/vendor/k8s.io/client-go/kubernetes \
    | xargs go list -f '{{if not .Standard}}{{.ImportPath}}{{end}}' \
    | grep pflag
```

/cc @sttts @kubernetes/sig-api-machinery-pr-reviews 

Edit: it looks like k8s.io/client-go/tools/clientcmd still imports this. So practically, if you're loading from a kubeconfig you're going to get the import. Might still be a good cleanup to make though.

```release-note
NONE
```