Remove all references to Third-Party AI features setting

What does this MR do and why?

• Issue for removing this setting: https://gitlab.com/gitlab-org/gitlab/-/issues/430890
• This MR removes all references to this setting in the UI, documentation, the code.
• /ml/ai-assist endpoint also no longer needed: #424771 (closed)
• Issue for removing third-party AI services claim for AI Gateway: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#325 (closed) (for now we are just hard-coding it to true because it is unused)
• The actual database columns will need to be removed in the next milestone

Screenshots or screen recordings

Code suggestions settings copy change per #430871 (closed)

Before	After

How to set up and validate locally

AI features can be used without the third-party group setting.

So you can follow the instructions here and skip the step to enable "Third-party AI services" (that setting isn't even visible in the UI or updateable via the API on this branch) and all features should work as expected

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.