Remove `new_graphql_users_autocomplete` feature flag

What does this MR do and why?

This fixes the issue list and issue board filter bars to use the new autocomplete endpoint that includes descendant group and project members.

This has been enabled on GitLab.com since August 17.

How to set up and validate locally

1. On the group issue list filter bar, try filtering by Author
2. You should see descendant project members even if they are not members of the parent group

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 #417757 (closed)