Remove relay node from heartwood.

No heartwood function creates Relay nodes. They are a consumer concern.

schema.go

@@ -29,10 +29,6 @@ func NewUserNode(pubkey string) *Node {
 	return NewNode("User", Properties{"pubkey": pubkey})
 }
 
-func NewRelayNode(url string) *Node {
-	return NewNode("Relay", Properties{"url": url})
-}
-
 func NewEventNode(id string) *Node {
 	return NewNode("Event", Properties{"id": id})
 }