Revert "Performance optimization suggestions"

This reverts commit f82d53f75d.

tsdb/index/inmem/inmem.go

@@ -1066,7 +1066,6 @@ func (i *Index) assignExistingSeries(shardID uint64, seriesIDSet *tsdb.SeriesIDS
 		} else {
 			// Add the existing series to this shard's bitset, since this may
 			// be the first time the series is added to this shard.
-			if !seriesIDSet.Contains(ss.ID) {
 			seriesIDSet.Lock()
 			if !seriesIDSet.ContainsNoLock(ss.ID) {
 				seriesIDSet.AddNoLock(ss.ID)
@@ -1074,7 +1073,6 @@ func (i *Index) assignExistingSeries(shardID uint64, seriesIDSet *tsdb.SeriesIDS
 			seriesIDSet.Unlock()
 		}
 	}
-	}
 	i.mu.RUnlock()
 	return keys[:n], names[:n], tagsSlice[:n]
 }