stand: Stop support booting 4.x and earlier kernels

FreeBSD 4.x and earlier used the bi_bios_geom to get the geometry of the
device. Starting in 5.x, with the wdc -> ata rewrite, it was used only
in pc98 kernels to report geometry of the drives. It can be safely
removed as booting kernels this old is no longer supported. This saves
176 bytes in the BIOS loader.

Sponsored by: Netflix
Reviewed by: adrian, emaste
Differential Revision: https://reviews.freebsd.org/D36543