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. 

  1. Create a new object (assignment, discussion, quiz, etc)
  2. Link the locked rubric to the new object and save. 
  3. Edit the new object and disassociate the rubric from it and save again.
  4. You should now be able to delete or edit the rubric.