Prevent further deletion of epic-issue relationships

What does this MR do and why?

!73007 (merged) accidentally destroyed epic-issue relationship records belonging to subgroup issues due to a slight mismatch in hierarchy checks.

This MR doesn't revert the changes entirely, but makes the migration a no-op since it has already run on gitlab.com.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #348477 (closed)