Fix Gitaly N+1 queries in related merge requests API

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9661, we removed the subscribed field from API endpoints that needed to return a list of issues or merge requests when to avoid full Markdown processing.

However, we introduced a performance regression in GitLab 11.10 (gitlab-foss!26367 (merged)) when we converted MergeRequestBasic to MergeRequest in order to expose the head pipeline in the /api/:version/projects/:id/issues/:issue_iid/related_merge_requests endpoint.

To avoid this overhead, we now omit the subscribed field in this API endpoint.