stand: Stop support booting 4.x and earlier kernels

[imp]

Summary

FreeBSD 4.x and earlier used the bi_bios_geom to get the geometry of the
device. Starting in 5.x it was used only in pc98 kernels to report
geometry of the drives. It can be safely removed, but place behind
LOADER_SUPPORT_PRE_5X_KERNEL for the moment. This saves 176 bytes in the
BIOS loader.

Sponsored by: Netflix

Test Plan

I'm pretty sure we no longer need this, and we don't really need to boot old kernels.
In an ideal world, we'd set some flag, but there's no flag value other than != 0
While we used to return the 1.2MB Floppy geometry for unknown / in valid
drives, that's not checked in any meaningful way in the kernels I looked at.
They are fine being 0 for any kernel any normal person (and like any extreme
special case / extreme use people), and there's no need to preserve them and
we wind up saving a few bytes in the process...

[imp]

question for reviewers: #ifdef or rm code?

[adrian]

i think it's fine to nuke it?