ZTS: remove empty zpool_add--allow-ashift-mismatch test
Added in b1e46f869, but empty, so no point keeping it around.
Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16931