Remove unnecessary lock in loadWAL (#6107)

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

tsdb/head.go

@@ -332,7 +332,6 @@ func (h *Head) loadWAL(r *wal.Reader, multiRef map[uint64]uint64) (err error) {
 	// read from the WAL are processed in order.
 	var (
 		wg      sync.WaitGroup
-		multiRefLock sync.Mutex
 		n       = runtime.GOMAXPROCS(0)
 		inputs  = make([]chan []record.RefSample, n)
 		outputs = make([]chan []record.RefSample, n)
@@ -394,9 +393,7 @@ func (h *Head) loadWAL(r *wal.Reader, multiRef map[uint64]uint64) (err error) {
 
 				if !created {
 					// There's already a different ref for this series.
-					multiRefLock.Lock()
 					multiRef[s.Ref] = series.ref
-					multiRefLock.Unlock()
 				}
 
 				if h.lastSeriesID < s.Ref {