Clean up role_targeted_broadcast_messages

What does this MR do and why?

This issue is to cleanup the role_targeted_broadcast_messages feature flag but KEEP THE BEHAVIOUR. It has been rolled out for over a year.

Screenshots or screen recordings

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How to set up and validate locally

• Sign in as root user. Go to /admin/broadcast_messages.
• Make sure there is Target roles section.

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 #399200 (closed)