consumer: don't check LastStableOffset

It's only needed for transactions (which we don't support yet) and the
logic was wrong anyway. Fixes slow consuming for certain Kafka 0.11
configurations.

consumer.go

@@ -546,11 +546,6 @@ func (child *partitionConsumer) parseRecords(block *FetchResponseBlock) ([]*Cons
 		} else {
 			incomplete = true
 		}
-
-		if child.offset > block.LastStableOffset {
-			// We reached the end of closed transactions
-			break
-		}
 	}
 
 	if incomplete || len(messages) == 0 {