acme/autocert: remove unused listener struct field
acme/autocert: remove unused listener struct field

Change-Id: Ie7f4848d7388811bdcf1937eab9d603de64d6e96
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/219220
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

golang/crypto,golang/crypto,golang/crypto