ata: libata: drop 'sas_last_tag'

Unused now.

Fixes: 4f1a22ee

 ("libata: Improve ATA queued command allocation")
Cc: John Garry <john.garry@huawei.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>