D2L currently has a bug logged for this problem and will have it resolved at some point in the future. However, this workaround will allow the rubric to be edited or removed.
- Create a new object (assignment, discussion, quiz, etc)
- Link the locked rubric to the new object and save.
- Edit the new object and disassociate the rubric from it and save again.
- You should now be able to delete or edit the rubric.