bsdinstall: Remove support for ZFS + MBR disk layouts

It hasn't worked for some time -- as reported in review D40816
"Installing FreeBSD with Auto ZFS + MBR has been broken ever since the
move to OpenZFS with FreeBSD 13." It relied on the partition table and
ZFS data overlapping in a very fragile way and is not a good idea.

Reviewed by: jhb
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51028