crypto/tls: remove leftover extensionNextProtoNeg constant

NPN was removed in CL 174329.

Change-Id: Ic63ad53e7e24872e28673d590727e0300f435619
Reviewed-on: https://go-review.googlesource.com/c/go/+/208224
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/tls/common.go

@@ -100,7 +100,6 @@ const (
 	extensionCertificateAuthorities  uint16 = 47
 	extensionSignatureAlgorithmsCert uint16 = 50
 	extensionKeyShare                uint16 = 51
-	extensionNextProtoNeg            uint16 = 13172 // not IANA assigned
 	extensionRenegotiationInfo       uint16 = 0xff01
 )