ipfw: Remove more unused IP_FW_* cases

All of the do_cmd() calls are in dummynet.c and specify the socket
option at compile time; none of these removed cases are used in ipfw
after the v3 work.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53378

(cherry picked from commit 0e2e0fb955adf15a217949bc4cc337d53d2c7259)