stand: Remove i386-only support fire firewire

Remove support for booting off of firewire, and for having dcons via
firewire in the loader. Kernel support for these things is unchanged.
Discussed on arch@ and the current state is not working (and the build
was wrong to boot).

Sponsored by: Netflix
Discussed: https://lists.freebsd.org/archives/freebsd-arch/2022-November/000267.html
Reviewed by: kevans, melifaro, emaste
Differential Revision: https://reviews.freebsd.org/D37334