Unlink should only show currently linked merge requests
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The following discussion from !207136 should be addressed:
- [ ] @mle started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/207136#note_2851627498): (+2 comments)
> @ptalbert **issue** `/unlink` should only show the currently linked merge requests
>
> In this merge request I only linked the "second change" merge request so that's the only one that should be listed.
> 
>
> For refernece, this functionality is correctly done in work items https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183102+
Also `/unlink` should not appear if there are no dependent merge requests.https://gitlab.com/gitlab-org/gitlab/-/merge_requests/207136#note_2851627500
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