Remove i386 XBOX support.

It is for console presented at 2001 and featuring Pentium III
processor. Even if any of them are still alive and run FreeBSD, we do
not have any sign of life from their users. While removing another
dozens of #ifdefs from the i386 sources reduces the aversion from
looking at the code and improves the platform vitality.

Reviewed by: cem, pfg, rink (XBOX support author)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D13016