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Remove deprecated --require-kubeconfig flag, remove default --kubeconfig value
**What this PR does / why we need it**:
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes#41161
**Special notes for your reviewer**:
**Release note**:
```release-note
Remove deprecated --require-kubeconfig flag, remove default --kubeconfig value
```
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.
Kubernetes Submit Queue