Remove ci_editor_ai_conversations table and related jobs

What does this MR do and why?

https://gitlab.com/gitlab-org/gitlab/-/issues/423634

An experimental feature related to AI is being removed. The application code (front+backend) has been removed and the related sidekiq workers have been no-op'ed in %16.7. The backend MR was !138448 (merged).

This MR removes the database table and the deprecated sidekiq workers.

https://docs.gitlab.com/ee/development/migration_style_guide.html#dropping-a-database-table

https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#in-a-subsequent-separate-minor-release

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