Remove broadcast messages from internal API.

What does this MR do?

The broadcast messages have been added to the post_receive endpoint in gitlab-foss!13916 (merged). So these endpoints are no longer used

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #33623 (closed)