Cleanup Code Suggestions related code

What does this MR do and why?

This relates to https://gitlab.com/gitlab-org/gitlab/-/issues/422657. It

1. removes Code Suggestions related code from Usage Quotas > Seats area
2. updates location for GraphQL Add-on related files (follow-up on private convo with @kpalchyk)
3. renames GraphQL Add-on related files to maintain consistency (addon to add_on)

There is no visual change.

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