ZTS: remove unused TESTDIRS from pam/cleanup.ksh

Remove TESTDIRS as it is not set for pam tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #16920