Don't create event in Merge Request Create Service

What does this MR do?

It does not create a new event in MergeRequests::CreateService because the event is created afterwards in the NewMergeRequestWorker.

It also contains a migration that removes duplicated records.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The creation of events in both service and worker caused duplicated events when a new MR is created

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #36465 (closed)