Handle distributed queries when shards != data nodes

Fixes #2272

There was previously a explict panic put in the query engine to prevent
queries where the number of shards was not equal to the number of data nodes
in the cluster.  This was waiting for the distributed queries branch to land
but was not removed when that landed.

tx.go

@@ -142,10 +142,6 @@ func (tx *tx) CreateMapReduceJobs(stmt *influxql.SelectStatement, tagKeys []stri
 
 			// create mappers for each shard we need to hit
 			for _, sg := range shardGroups {
-				if len(sg.Shards) != 1 { // we'll only have more than 1 shard in a group when RF < # servers in cluster
-					// TODO: implement distributed queries.
-					panic("distributed queries not implemented yet and there are too many shards in this group")
-				}
 
 				shard := sg.Shards[0]