Release chat notification branch filter for deployments

What does this MR do and why?

Removes the chat_notification_deployment_protected_branch_filter feature flag, finalizing the work on #217020 (closed)

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 #349131 (closed)