From the ConfD UG Chapter 15 "The NETCONF Server":
For the <delete-config> operation specified in RFC 4741 / RFC 6241, only <url> with scheme "file" is supported for the <target> parameter - i.e. no data stores can be deleted. The concept of deleting a data store is not well defined, and at odds with the transaction-based configuration management of ConfD. To delete the entire contents of a data store, with full transactional support, a <copy-config> with an empty <config/> element for the <source> parameter can be used.