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kubelet: remove the --docker-exec-handler flag

Stop supporting the "nsenter" exec handler. Only the Docker native exec
handler is supported.

The flag was deprecated in Kubernetes 1.6 and is safe to remove
in Kubernetes 1.9 according to the deprecation policy.

**What this PR does / why we need it**:

**Which issue this PR fixes** : fixes #40229

**Special notes for your reviewer**:
N/A

**Release note**:

```release-note
Remove the --docker-exec-handler flag. Only native exec handler is supported.
```