Use a single flag for AI experimentation endpoints

What does this MR do and why?

• One feature flag ensures that these are only usable for GitLab team members. No need for two.
• Removes anthropic_experimentation feature flag entirely. The openai_experimentation feature flag is also used in other places so that will be removed separately, see: https://gitlab.com/gitlab-org/gitlab/-/issues/423979
• Removed the secondary flag for Vertex here: !130598 (merged)
• #423941 (closed)
• #409939 (closed)

How to set up and validate locally

Endpoints documented here: https://docs.gitlab.com/ee/development/ai_features/index.html#experimental-rest-api

Should now be available with just the ai_experimentaton_endpoints feature flags enabled.

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.