libzfs: Remove unused zpool_get_physpath()

This is an oddly specific function that has never had any consumers in
the history of this repo. Get rid of it and the pile of helper
functions that exist for it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #13724