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.