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

Change-Id: I6ba8d07b9e53b01f25f4c1c8eac629aaa47de3a1
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/664836
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

acme/http.go

@@ -66,7 +66,7 @@ func (c *Client) retryTimer() *retryTimer {
 // The n argument is always bounded between 1 and 30.
 // The returned value is always greater than 0.
 func defaultBackoff(n int, r *http.Request, res *http.Response) time.Duration {
-	const max = 10 * time.Second
+	const maxVal = 10 * time.Second
 	var jitter time.Duration
 	if x, err := rand.Int(rand.Reader, big.NewInt(1000)); err == nil {
 		// Set the minimum to 1ms to avoid a case where
@@ -86,10 +86,7 @@ func defaultBackoff(n int, r *http.Request, res *http.Response) time.Duration {
 		n = 30
 	}
 	d := time.Duration(1<<uint(n-1))*time.Second + jitter
-	if d > max {
-		return max
-	}
-	return d
+	return min(d, maxVal)
 }
 
 // retryAfter parses a Retry-After HTTP header value,