Inside of the Rubric Manager's Select Tab you can see all of your active rubrics. If there is a rubric you no longer use, you can archive that rubric to hide it from the My Rubrics section of the Select Tab
To archive a rubric, navigate to the Create & Edit Tab of the Rubric Manager, scroll to the bottom of the rubric and you will find an Archive this Rubric button.
After you click this, you will see in the Select Tab that the rubric is no longer shown under My Rubrics
To find a Rubric and Unarchive it you click on the [+] to see all of your archived Rubrics.