multi: remove kvdb.RTx from ForEachNodeChannel

Since we have not removed all call-sites that make use of this
parameter, we can remove it. This helps hide DB-specific details from
the interface we will introduce for the graph store.

graph/db/kv_store.go

@@ -3103,10 +3103,15 @@ func nodeTraversal(tx kvdb.RTx, nodePub []byte, db kvdb.Backend,
 //
 // Unknown policies are passed into the callback as nil values.
 func (c *KVStore) ForEachNodeChannel(nodePub route.Vertex,
-	cb func(kvdb.RTx, *models.ChannelEdgeInfo, *models.ChannelEdgePolicy,
+	cb func(*models.ChannelEdgeInfo, *models.ChannelEdgePolicy,
 		*models.ChannelEdgePolicy) error) error {
 
-	return nodeTraversal(nil, nodePub[:], c.db, cb)
+	return nodeTraversal(nil, nodePub[:], c.db, func(_ kvdb.RTx,
+		info *models.ChannelEdgeInfo, policy,
+		policy2 *models.ChannelEdgePolicy) error {
+
+		return cb(info, policy, policy2)
+	})
 }
 
 // ForEachSourceNodeChannel iterates through all channels of the source node,