CI cleanup: remove onlineddl_declarative, onlineddl_singleton (cleanup part 2)

[shlomi-noach]

Description

Complementing #12061 and #12055, in this PR we completely remove onlineddl_declarative and onlineddl_singleton. The actual tests themselves are now incorporated in onlineddl_declarative.

This removes 4 CI workflows.

I have already removed the Required constraint from these tests in the main branch protection rules. Let's merge this before v16 so that we have a cleaner and smaller CI workflow set.

Related Issue(s)

General tracking issue, #6926

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[mattlord]

Thank you! 🙇 I appreciate it.