So instead there's an archive feature. Archiving doesn't alter the rubric in any way, it just basically hides it out of view. But you can always go back to it via the Archived Rubrics area.
To archive a rubric:
Log in and click on the "my rubrics" link at the top right.
You'll see a list of all of your active rubrics along with an "archive" link to the right. Click "archive" and follow the prompts.