Kernel module shim sources have no business being in the userland
build directory, especially those for other operating systems.
The kernel module shims for other operating systems are hereby removed.
The kernel module shim for FreeBSD, mlfk_ipl.c, is already in
sys/contrib/ipfilter/netinet. The one here is never used and should
not be in the userland build directory either.

mlfk_rule.c isn't used either however we will keep it in case someone
wishes to use this shim to load rules via a kernel module, handy for
embedded. In that case it should be copied to
sys/contrib/ipfilter/netinet and a Makefile created to employ it.
(Probably a useful documentation project when time permits.)

MFC after: 1 month