scsi: libsas: Drop SAS_TASK_AT_INITIATOR

This flag is now only ever set, so delete it.

This also avoids a use-after-free in the pm8001 queue path, as reported in
the following:

https://lore.kernel.org/linux-scsi/c3cb7228-254e-9584-182b-007ac5e6fe0a@huawei.com/T/#m28c94c6d3ff582ec4a9fa54819180740e8bd4cfb

https://lore.kernel.org/linux-scsi/0cc0c435-b4f2-9c76-258d-865ba50a29dd@huawei.com/

[mkp: checkpatch + two SAS_TASK_AT_INITIATOR references]

Link: https://lore.kernel.org/r/1644489804-85730-3-git-send-email-john.garry@huawei.com
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>