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.

rpcserver.go

@@ -6345,7 +6345,7 @@ func (r *rpcServer) GetNodeInfo(ctx context.Context,
 	// With the public key decoded, attempt to fetch the node corresponding
 	// to this public key. If the node cannot be found, then an error will
 	// be returned.
-	node, err := graph.FetchLightningNode(nil, pubKey)
+	node, err := graph.FetchLightningNode(pubKey)
 	switch {
 	case err == channeldb.ErrGraphNodeNotFound:
 		return nil, status.Error(codes.NotFound, err.Error())
@@ -7393,7 +7393,7 @@ func (r *rpcServer) ForwardingHistory(ctx context.Context,
 			return "", err
 		}
 
-		peer, err := r.server.graphDB.FetchLightningNode(nil, vertex)
+		peer, err := r.server.graphDB.FetchLightningNode(vertex)
 		if err != nil {
 			return "", err
 		}