Remove backend components of automatic AI summary feature
What does this MR do and why?
We have stop using the automatic diff summary, removed frontend usage and can now safely remove it from the backend code.
To Test:
- Turn on feature flag: add_ai_summary_for_new_mr
- Create a new MR
- Ensure we can still summarize a new merge request
Related to #451241 (closed)
Edited by Marc Shaw