Navigated here:
https://www.dropbox.com/backup/all
I see the backup I want to delete; it shows "Unlinked".
I click the "Delete" button, click "continue" on the dialog, and it shows "Deleting "XX" backup from Dropbox" at the bottom for a minute or so, and then that goes away, but the backup remains.
Tried deleting it that way several times and it is still there.
Edit: after about 30 minutes from the first delete, it appears to finally be gone.