Automatic merge from submit-queue (batch tested with PRs 60363, 59208, 59465, 60581, 60702). 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>.
apiserver clean code
**What this PR does / why we need it**:
1. clean up some redundant code in kube-apiserver startup
1. comment on `preparedGenericAPIServer`, which is just a wrapper of `GenericAPIServer`.
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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