sync: remove unused const

Change-Id: I4382b5317cf7f39a48005516ff6d437883c6fd07
Reviewed-on: https://go-review.googlesource.com/c/go/+/456495
Reviewed-by: Dominik Honnef <dominik@honnef.co>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Changkun Ou <mail@changkun.de>

src/sync/map_bench_test.go

@@ -389,7 +389,6 @@ func BenchmarkCompareAndSwapNoExistingKey(b *testing.B) {
 }
 
 func BenchmarkCompareAndSwapValueNotEqual(b *testing.B) {
-	const n = 1 << 10
 	benchMap(b, bench{
 		setup: func(_ *testing.B, m mapInterface) {
 			m.Store(0, 0)