ZTS: runfiles: remove explicit outputdir

The config file value overrides any set by the operator, making it quite
difficult to put the test output elsewhere. The default is
/var/tmp/test_results (via BASEDIR in test-runner) so this shouldn't
change anything for the default case.

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>