To streamline your audience list and make it easier to navigate, you have the ability to archive any audience you no longer need, provided that it is not selected in any active campaigns.

Starting in the Audiences menu, under the Current  tab, click the 3 dots to the right of the audience you would like to archive.

Choose Archive.

You can also reinstate an audience at any time. 
Under the Archived  tab, click the 3 dots to the right of the audience you would like to reinstate.

Choose Unarchive.