Remove AI experimentation endpoints

What does this MR do and why?

• These are not used.
• Were turned on only for gitlab team members, but have not heard of anyone using them in the last 8 months (since we last discussed getting rid of these) so letting these endpoints around is an unnecessary liability
• https://gitlab.com/gitlab-org/gitlab/-/issues/406754

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

This is removal of an experimentation endpoint, there is nothing to validate.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.