EpicIssue has an unexpected default relative position

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31351 broke our EE specs. We assume that a NULL relative position will be updated to the expected value.

But for epic_issues have a default relative position, so the spec doesn't get the test fixtures we were assuming it does.

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14956 I'm skipping those specs for now. Should we solve that differently?

---

We should remove the default value for epic_issues.relative_position because it is not needed anyway. We set the position explicitly when creating one in https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/services/epic_issues/create_service.rb#L21.

After removing the default, we can then remove the skip lines added in !14956 (merged)