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

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd