Automatic merge from submit-queue
Remove kubectl namespace command
**What this PR does / why we need it**:
It removes deprecated `kubectl namespace` command, which `has been superseded by the context.namespace field of .kubeconfig files. See 'kubectl config set-context --help' for more details`. It was done nearly two years ago, so like `// TODO remove once people have been given enough time to notice` comment says it may be a good time to get rid of it.
**Special notes for your reviewer**:
None ATM.
**Release note**:
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```release-note
Remove kubectl namespace command
```
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