refactor: create FetchLightningNode with no tx param

In preparation for adding a clean Graph DB interface, we create a
version of FetchLightningNode that doesnt allow a caller to provide in a
transaction.

routing/router.go

@@ -2915,7 +2915,7 @@ func (r *ChannelRouter) GetChannelByID(chanID lnwire.ShortChannelID) (
 func (r *ChannelRouter) FetchLightningNode(
 	node route.Vertex) (*channeldb.LightningNode, error) {
 
-	return r.cfg.Graph.FetchLightningNode(nil, node)
+	return r.cfg.Graph.FetchLightningNode(node)
 }
 
 // ForEachNode is used to iterate over every node in router topology.