Just found it in the Api docs. Delete seems to throw though:

CREATE will create the relationship only if it doesn't exist, and error otherwise.

TOUCH will upsert the relationship, and will not error if it already exists.

DELETE will delete the relationship and error if it doesn't exist.

I've been wondering why there isn't an idempotent DELETE for tuples. It seems like it would be a nice complement to TOUCH. I've seen some questions before but never discussion on if something is in the works or if there is a reason it isn't supported