multi+refactor: let FetchChanInfos not take tx

In preparation for having a clean graph DB interface, refactor
FetchChanInfos so that no transaction can be provided.

routing/router.go

@@ -1023,7 +1023,7 @@ func (r *ChannelRouter) pruneZombieChans() error {
 		}
 
 		disabledEdges, err := r.cfg.Graph.FetchChanInfos(
-			nil, disabledChanIDs,
+			disabledChanIDs,
 		)
 		if err != nil {
 			return fmt.Errorf("unable to fetch disabled channels "+