acme/autocert: remove unused listener struct field

Change-Id: Ie7f4848d7388811bdcf1937eab9d603de64d6e96
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/219220
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

acme/autocert/listener.go

@@ -72,7 +72,6 @@ func NewListener(domains ...string) net.Listener {
 // the Manager m's Prompt, Cache, HostPolicy, and other desired options.
 func (m *Manager) Listener() net.Listener {
 	ln := &listener{
-		m:    m,
 		conf: m.TLSConfig(),
 	}
 	ln.tcpListener, ln.tcpListenErr = net.Listen("tcp", ":443")
@@ -80,7 +79,6 @@ func (m *Manager) Listener() net.Listener {
 }
 
 type listener struct {
-	m    *Manager
 	conf *tls.Config
 
 	tcpListener  net.Listener