Removed feature flag `ff_external_audit_events`

What does this MR do and why?

Removed feature flag ff_external_audit_events which enabled feature for enabling instance level external audit event destinations.

Rollout issue: #393772 (closed)

How to set up and validate locally

All the commands mentioned in https://docs.gitlab.com/ee/administration/audit_event_streaming/#instance-streaming-destinations should be working as expected and you should be receiving audit events on the added external destination.

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 #417708 (closed)