Automatic merge from submit-queue. 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>.

Cleanup apiserver errors

- delete unused methods (is this safe, or might they be used in another project outside kubernetes/kubernetes?)
- use the standard library `http.Error` function (functional change adds a newline)

```release-note
NONE
```

/sig api-machinery
/kind cleanup