Merge pull request #33275 from maciaszczykm/remove-kubectl-namespace-cmd

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It removes deprecated `kubectl namespace` command, which `has been superseded by the context.namespace field of .kubeconfig files.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
```

docs/man/man1/kubectl-namespace.1

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.