Remove MVP implementation of CI config chat

What does this MR do and why?

This MR removes the backend code for the MVP implementation of CI config chat. The feature was experimentally implemented and never released.

The following sidekiq workers are no-op'ed so they can be removed in the next milestone https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#removing-worker-classes.

• ee/app/workers/ci/llm/generate_config_worker.rb
• ee/app/workers/open_ai/clear_conversations_worker.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #423634