Stop supporting kubeflow/mxjob integration

[tenzen-y]

What would you like to be cleaned:
We should stop supporting the kubeflow/mxjob integration.

Why is this needed:
Due to Apache MXNet having been archived, the kubeflow community plans to remove MXJob from the training-operator.

kubeflow/training-operator#1953

[tenzen-y]

We need to watch #1953, and then we can decide when we stop supporting this integration.

[terrytangyuan]

Yes, I don't have any concerns about dropping Apache MXNet support.

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all issues.

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• After 90d of inactivity, lifecycle/stale is applied
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/lifecycle stale

[tenzen-y]

/lifecycle frozen.

Kubeflow Community decided to remove MXJob after 1 minor release.
So, we can remove the kubefow/mxjob integration after it.

[tenzen-y]

/lifecycle frozen