!chore: Remove support for MXJob

[terrytangyuan]

Unfortunately, Apache MXNet has been archived. This PR removes MXJob support.

[google-oss-prow]

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[github-actions]

Pull Request Test Coverage Report for Build 7023065747

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.9%) to 41.988%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	6	80.67%

Totals
Change from base Build 7007158786:	-0.9%
Covered Lines:	3346
Relevant Lines:	7969

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 7023661984

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.9%) to 41.975%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob.go	1	91.06%

Totals
Change from base Build 7007158786:	-0.9%
Covered Lines:	3345
Relevant Lines:	7969

---

💛 - Coveralls

[terrytangyuan]

This would be a breaking change but I think it's relatively safe given how few people are using MXNet these days. cc @kubeflow/wg-training-leads

[andreyvelich]

Thank you for creating this @terrytangyuan.
That is sad to hear that Apache MXNet has been archived, it has a lot of potential.

I think, we still use MXNet for various things. E.g. this Trial image is the most popular in Katib: https://github.com/kubeflow/katib/tree/master/examples/v1beta1/trial-images/mxnet-mnist

Let's discuss it in one of our upcoming WG meetings on how to inform Kubeflow users about it.
/hold

[terrytangyuan]

Any concerns of removing this? I don't think we want to continue supporting it.

[andreyvelich]

Related: #1996.
We should merge this PR once we make Training Operator 1.9 release.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[tenzen-y]

/remove-lifecycle stale

[terrytangyuan]

Does anyone want to pick this up?