qla2xxx: Remove racy, now-redundant check of sess_tearing_down

Now that target_submit_cmd() / target_get_sess_cmd() check
sess_tearing_down before adding commands to the list, we no longer
need the check in qlt_do_work().  In fact this check is racy anyway
(and that race is what inspired the change to add the check of
sess_tearing_down to the target core).

Cc: Chad Dupuis <chad.dupuis@qlogic.com>
Cc: Arun Easi <arun.easi@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>