The error message in zpool_vdev_remove() said top-level devices
could be removed, but that has never been true.
Reported-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes#4506Closes#5213