[UI] Remove Deprecated Katib UI

[andreyvelich]

We created our new Katib UI almost 2.5 years ago: #1427.
This UI has a lot of great features and it is well adopted by many Katib users.
We can remove the legacy Katib UI and continue improving the new one.

I will close this project: https://github.com/kubeflow/katib/projects/1 also.
Let's track the UI features in the separate issues.

Also, I removed prow_config, since we are not longer using prow for our E2Es.

Thank you for your long-term support and delivering such great features for our Kubeflow users!
@kimwnasptd @elenzio9 @tenzen-y @johnugeorge @apo-ger @d-gol @seong7 @a9p

I am looking forward to improve Katib user experience further.

[google-oss-prow]

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[tenzen-y]

Thanks! @andreyvelich
/lgtm