ipfw: Retire obsolete compat code

The current IPFW version 3 dates to 2010 (commit cc4d3c30ea28, "Bring in
the most recent version of ipfw and dummynet, developed").

The compat code for FreeBSD 8 and earlier has a number of issues and is
no longer needed, so remove it.

Approved by: re (cperciva)
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: ae, glebius
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53343

(cherry picked from commit c59aab9a5b3970b3ccec744f759e6cb87e938dbe)
(cherry picked from commit 9657c50cdd7741404d99881fdd9243175086ede1)