channeldb/graph: don't create node bucket in DeleteChannelEdge

Instead return ErrGraphNodeNotFound directly. If the node bucket was
created it would be empty, and the call delChannelByEdge ->
fetchChanEdgePolicies -> fetchChanEdgePolicy ->
deserializeChanEdgePolicy -> fetchLightningNode would return this error
anyway.

channeldb/graph.go

@@ -1083,9 +1083,10 @@ func (c *ChannelGraph) DeleteChannelEdge(chanPoint *wire.OutPoint) error {
 		if chanIndex == nil {
 			return ErrEdgeNotFound
 		}
-		nodes, err := tx.CreateBucketIfNotExists(nodeBucket)
-		if err != nil {
-			return err
+
+		nodes := tx.Bucket(nodeBucket)
+		if nodes == nil {
+			return ErrGraphNodeNotFound
 		}
 
 		return delChannelByEdge(