add delete-orphan to Vendor.contacts relationship

(cherry picked from commit 3fd6c9212e)

rattail/db/extension/model.py

@@ -641,7 +641,8 @@ class Vendor(Base):
 
     contacts = relationship(VendorContact, backref='vendor',
                             collection_class=ordering_list('preference', count_from=1),
-                            order_by=VendorContact.preference)
+                            order_by=VendorContact.preference,
+                            cascade='save-update, merge, delete, delete-orphan')
 
     def __repr__(self):
         return "<Vendor: %s>" % self.name