Resource: Change SubOnlyExisting() to same signature as AddOnlyExisting()

The AddOnlyExisting function takes two resource objects and returns a new object. The SubOnlyExisting method is called on a resource receiver modifying the receiver object. 

These two should use the same kind of signature taking two resource objects and returning a new object. In most use cases for SubOnlyExisting we do a clone before we call within a locked method on an object that contains the resource.

This clone becomes obsolete when we make the change.