channeldb: don't initialise edge index bkt as top level

The `edgeIndexBucket` key name is used to key a sub-bucket within the
`edgeBucket` bucket. And so the `edgeIndexBucket` is not a top level
bucket.

channeldb/graph.go

@@ -314,7 +314,6 @@ func (c *ChannelGraph) getChannelMap(edges kvdb.RBucket) (
 var graphTopLevelBuckets = [][]byte{
 	nodeBucket,
 	edgeBucket,
-	edgeIndexBucket,
 	graphMetaBucket,
 }