internal/timeseries: use built-in max/min to simplify the code

Change-Id: I3d408025af072559cac1f2b403a90518995d0a5a
Reviewed-on: https://go-review.googlesource.com/c/net/+/664855
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: cuishuang

internal/timeseries/timeseries_test.go

@@ -161,10 +161,3 @@ func TestExpectedErrorRate(t *testing.T) {
 		checkNear(t, ts.Latest(2, buckets), min(float64(i), 3600), 600)
 	}
 }
-
-func min(a, b float64) float64 {
-	if a < b {
-		return a
-	}
-	return b
-}