acme/autocert: Remove unused retryAfter func

Change-Id: I607d3d2c28aa5a125bddc3fea4f9508942bc1365
Reviewed-on: https://go-review.googlesource.com/93315
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

acme/autocert/autocert.go

@@ -27,7 +27,6 @@ import (
 	"net"
 	"net/http"
 	"path"
-	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -942,16 +941,6 @@ func validCert(domain string, der [][]byte, key crypto.Signer) (leaf *x509.Certi
 	return leaf, nil
 }
 
-func retryAfter(v string) time.Duration {
-	if i, err := strconv.Atoi(v); err == nil {
-		return time.Duration(i) * time.Second
-	}
-	if t, err := http.ParseTime(v); err == nil {
-		return t.Sub(timeNow())
-	}
-	return time.Second
-}
-
 type lockedMathRand struct {
 	sync.Mutex
 	rnd *mathrand.Rand