Automatic merge from submit-queue (batch tested with PRs 58777, 58978, 58977, 58775). 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>.
Removal of KubeletConfigFile feature gate: Step 3 (final)
This PR completes the work started in
https://github.com/kubernetes/kubernetes/pull/58760
by completely removing the KubeletConfigFile feature gate.
We stopped setting the gate in test-infra in
https://github.com/kubernetes/test-infra/pull/6490.
```release-note
The alpha KubeletConfigFile feature gate has been removed, because it was redundant with the Kubelet's --config flag. It is no longer necessary to set this gate to use the flag. The --config flag is still considered alpha.
```
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