Remove AiProxyHelper and related code

What does this MR do and why?

We stop supporting PAT auth for Code Suggestions from %16.4
We should remove related code.

The changes in this MR affect Self-Managed only (from %16.4) — no changes for .com or .org (SaaS).

DB column (::Gitlab::CurrentSettings.ai_access_token) will be deprecated separately in multiple steps. It is tracked in #422174 (closed)

How to set up and validate locally

It removes no-op code, so we could only test that we didn't break any existing flow.
We describe how to test Self Managed Code Suggestions, for example, here: !128997 (merged)

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 #422379 (closed)