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>.

Removal of KubeletConfigFile feature gate: Step 1

This feature gate was redundant with the `--config` flag, which already
enables/disables loading Kubelet config from a file.

Since the gate guarded an alpha feature, removing it is not a violation
of our API guidelines.

Some stuff in `kubernetes/test-infra` currently sets the gate,
so removing will be a 3 step process:
1. This PR, which makes the gate a no-op.
2. Stop setting the gate in `kubernetes/test-infra`.
3. Completely remove the gate (this PR will get the release note).

```release-note
NONE
```