Refactor unused argument from gitlab_duo_available?

This is related to #439075 and depends on !156961 (merged).

What does this MR do and why?

After removing the hamilton_seat_management feature flag on !156961 (merged), we can refactor the deprecated namespace argument from CodeSuggestionsHelper#gitlab_duo_available?. That's what this MR does.

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

N/A

How to set up and validate locally

This is a refactor without any change in the current behavior. A green CI is enough verification.