I was doing some cleanup work at a customer site and noticed they had quite a few invalid servers listed in the "Known license servers" list when I was specifying the TS/RDS CAL server. I didn't find many references, so here's the procedure:

Remove the RDS Licensing Role Service, which should remove the server from the list. Likely the service has already been removed, and the server is gone, and thus why you are looking for another way to clean the list. If the server is no longer available, launch Active Directory Users and Computers, and set the option from the View menu labeled "Users, Contacts, and Computers as containers". I also select "Advanced Features" so I can see the Object tab. Find the object in ADUC within the OU structure, not using the Find tool. Expand the object and delete the sub-object named "TermServLicensing". This will remove it from the discovery list. You can delete the entire object if the server is gone.