Remove self_managed_code_suggestions_completion_api FF

What does this MR do and why?

⚠ This is for %16.4. Do not merge until the final commit that goes into %16.3 is announced

Full feature flag rollout - removing it from the codebase.

It does not affect SaaS (was never enabled there), and it is default enabled for Self-Managed starting from %16.3: !128480 (merged)

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 #418795 (closed)