Remove whole-disk vdev support from zfsboot

This is consistent with the removal of whole-disk vdev support from
libsa/zfs/zfs.c in r342151, and is part way to having the LBAs read
during probe be fully constrained by partition tables when present.

Reviewed by: tsoome
MFC after: 3 months
Differential Revision: https://reviews.freebsd.org/D19142