Remove unused variable in vdev_add_child()

Commit c3520e7 restructured vdev_add_child() in such a way that
the spa variable was unused during non-debug builds. This is
consistent with the upstream illumos code but because ZoL, unlike
illumos, is built with all compiler warnings enabled this causes
a legitimate warning. Revert this hunk of the patch to keep the
build clean.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3432