Fix double title in merge request chat messages

What does this MR do?

Remove redundant title from merge request chat messages.

EE port in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7370

What are the relevant issue numbers?

Closes #44586 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides