Automatic merge from submit-queue (batch tested with PRs 62486, 62471, 62183). 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>.

sarapprover: remove self node cert

The functionality to bootstrap node certificates is ready but is blocked by a separable issue discussed in: https://github.com/kubernetes/community/pull/1982. The functionality could be useful for power users who want to write their own approvers if the feature could be promoted to beta. In it's current state this feature doesn't help anybody.

I propose that we remove automated approval of node serving certificates for now and work towards getting the node functionality to beta.

cc @awly @kubernetes/sig-auth-pr-reviews 
```release-note
Remove alpha functionality that allowed the controller manager to approve kubelet server certificates.
```