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It removes deprecated `kubectl namespace` command, which `has been superseded by the context.namespace field of .kubeconfig files.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**:
<!--  Steps to write your release note:
1. Use the release-note-* labels to set the release note state (if you have access) 
2. Enter your extended release note in the below block; leaving it blank means using the PR title as the release note. If no release note is required, just write `NONE`. 
-->
```release-note
Remove kubectl namespace command
```