Remove references to hamilton_seat_management feature flag

This is part of #439075.

What does this MR do and why?

This MR removes the code references to the hamilton_seat_management feature flag. There is an additional refactoring to be made which is removing the namespace argument from CodeSuggestionsHelper#gitlab_duo_available? (here we just deprecate it). I've decided to do that on a separate MR so this one is only about retiring the feature flag.

MR acceptance checklist

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Screenshots or screen recordings

N/A

How to set up and validate locally

A green CI should be enough.