ipv4: remove unused const maxHeaderLen

It's unused since CL 162598.

Change-Id: I434dbc9950bb535167c40498ed7c15589db74eaf
Reviewed-on: https://go-review.googlesource.com/c/net/+/232537
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

ipv4/header.go

@@ -14,9 +14,8 @@ import (
 )
 
 const (
-	Version      = 4  // protocol version
-	HeaderLen    = 20 // header length without extension headers
-	maxHeaderLen = 60 // sensible default, revisit if later RFCs define new usage of version and header length fields
+	Version   = 4  // protocol version
+	HeaderLen = 20 // header length without extension headers
 )
 
 type HeaderFlags int