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
(https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/26367) 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.

Closes https://gitlab.com/gitlab-org/gitlab/issues/32948