Resolve "Incorrect default merge request title when Jira activated"

What does this MR do?

Stops setting a silly title of 'Resolves External Issue 123' for MRs created where the branch name starts with an external issue ID.

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

Don't think so!

Why was this MR needed?

This was always silly, but it became much more frequent in 10.3 as part of a refactor in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13884, so this is a 10.3 regression.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/36669.