There are two options to remove Backup projects that are no longer needed
Option 1.
Go to the Backup Tab and on the right side of the screen in the Backup Navigation panel > right-click the project name and select Delete.
Option 2.
If you can't delete your backup project from the Backup Navigation panel. Go to the backup location and look for a folder with the naming pattern "ARCHIVE-projectName" and a folder that ends with _archiveData (i.e. ARCHIVE-projectName_archiveData).
the Archive-projectName is your backup Project and archiveData contains you backup data files (* important if you delete these folders you will delete your backup along with your content).