fs/accounting: add option to delete stats

Removed PruneAllTransfers because it had no use. startedTransfers are
set to nil in ResetCounters.

fs/accounting/stats_test.go

@@ -431,25 +431,3 @@ func TestPruneTransfers(t *testing.T) {
 		})
 	}
 }
-
-func TestPruneAllTransfers(t *testing.T) {
-	const transfers = 10
-
-	s := NewStats()
-	for i := int64(1); i <= int64(transfers); i++ {
-		s.AddTransfer(&Transfer{
-			startedAt:   time.Unix(i, 0),
-			completedAt: time.Unix(i+1, 0),
-		})
-	}
-
-	s.mu.Lock()
-	assert.Equal(t, transfers, len(s.startedTransfers))
-	s.mu.Unlock()
-
-	s.PruneAllTransfers()
-
-	s.mu.Lock()
-	assert.Empty(t, s.startedTransfers)
-	s.mu.Unlock()
-}