scsi: qla2xxx: Remove unused del_sess_list field

"struct qla_tgt"->del_sess_list is no longer used since commit
726b8548 ("qla2xxx: Add framework for async fabric discovery").

Link: https://lore.kernel.org/r/0c335e86-5624-b599-5137-f1377419fb0c@I-love.SAKURA.ne.jp


Tested-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>