Delete custom notebook image

[jiripetrlik]

We stoped building this image. I think we can delete it completely.

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[ChristianZaccaria]

This removal is awesome, thanks!

There are a couple more things which could be done in the scope of this PR:

• Remove nightly-image-build.yaml workflow.
• Update the Manual Releases section in the readme. TBH, I think we should remove that whole section and rely solely on Automated Releases.

[jiripetrlik]

@ChristianZaccaria
Thank you for spotting this! Both points were removed.

I don't have any strong opinion whether to remove also whole readme for manual releases. But this PR is about custom notebook image. Therefore I think to remove Readme is for different PR.

[ChristianZaccaria]

@jiripetrlik sounds good! The removal of the Manual Release section could be made in a separate PR.

Wondering why the e2e is failing here? - I think it's unrelated, maybe it has to do with the faulty ray image being used in the SDK. There will be a fix soon.

[ChristianZaccaria]

/lgtm

[openshift-merge-robot]

PR needs rebase.

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