Remove include_descendant_shares_in_user_autocomplete feature flag

What does this MR do and why?

Includes descendant group and project members that come from shared groups in user autocomplete

This has already been enabled on GitLab.com for a week

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 #419669 (closed)