This is the last commit of the plan to remove the
GitlabIssueTrackerService, see:
https://gitlab.com/gitlab-org/gitlab/-/issues/218526#note_380963641

1. Remove the records in a post-migration
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35221
2. Remove the GitLabIssueTrackerService class
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36063
3. Run the migration again to remove any existing records in a
   post-migration. This commit!