Remove duplicates from issue related merge requests

What does this MR do?

Remove duplicates returned by Issues#related_merge_requests API that relies on ReferencedMergeRequestsService which returns 2 arrays one of related MRs and one of related MRs that close the issue(i.e. a subset of first one). We only need related MRs in this case so just pick the first array.

Does this MR meet the acceptance criteria?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57662

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

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team