Remove the is_physical_device ATF variable from the ZFS test suite

It's a holdover from the Solaris days. Frankly, I'm not 100% sure why it
exists. It seems like the purpose is to skip tests that partition disks
from running with file-backed vdevs. However, that's kind of useless since
the only reason to partition disks is to get a greater number, and
file-backed vdevs are unlimited. A better solution would be to remove
partitioning from all tests; if the system has insufficient disks then just
skip the tests. Besides, FreeBSD can always use vnode-backed md(4) devices,
which are unlimited in number and can be partitioned.

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