ipv4: remove unused const maxHeaderLen
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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

golang/net