Remove /ml/ai-assist

Problem

/ml/ai-assist was initially used for the Model Gateway to authorize a PAT granted by gitlab.com. However, this type of authentication has been removed in the meantime (see #286 (closed) ). Keeping /ml/ai-assist around at this point will only create confusion on how it is used or lead to attempts to use the endpoint for unintended usage (see gitlab-vscode-extension#750 (closed)).

Looking at the logs there are a very small number of requests to this endpoint (8 in 24 hours), so it should be safe to remove

Desired Outcome

/ml/ai-assist is completely removed from the codebase.

To see where it is defined, see the MR where it was added.