Remove Domain (Classic Site)

Even though you cannot edit the old site, you may be able to remove the domain from it.

Has the site been deleted?

https://sites.google.com/site/sites/system/app/pages/meta/dashboard/deleted 

If so you should be able to restore it and remove the domain.

Alternatively you can permanently delete it and the domain should be released within a day or so. My recommendation is to restore and remove.

Thanks Perjury!