crypto/tls: remove typeNextProtocol

Was this strictly necessary? No.
Did this deserve its own CL? Maybe not.
But I have a personal vendetta against NPN.

Change-Id: Ide1ad1092259dc23e3ead5c1d5269fc5cb2793d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/587275
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

common.go

@@ -92,7 +92,6 @@ const (
 	typeFinished            uint8 = 20
 	typeCertificateStatus   uint8 = 22
 	typeKeyUpdate           uint8 = 24
-	typeNextProtocol        uint8 = 67  // Not IANA assigned
 	typeMessageHash         uint8 = 254 // synthetic message
 )