multi: remove the need for NewRoutingGraph

The `graphsession.NewRoutingGraph` method was used to create a
RoutingGraph instance with no consistent read transaction across calls.
But now that the ChannelGraph directly implements this, we can remove
The NewRoutingGraph method.

server.go

@@ -1073,7 +1073,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.chanRouter, err = routing.New(routing.Config{
 		SelfNode:           selfNode.PubKeyBytes,
-		RoutingGraph:       graphsession.NewRoutingGraph(dbs.GraphDB),
+		RoutingGraph:       dbs.GraphDB,
 		Chain:              cc.ChainIO,
 		Payer:              s.htlcSwitch,
 		Control:            s.controlTower,