libata: remove ATA_FLAG_LOWTAG

sata_sil24 for some reason pukes when tags are allocated round-robin
which helps tag ordered controllers.  To work around the issue,
72dd299d5039 ("libata: allow sata_sil24 to opt-out of tag ordered
submission") introduced ATA_FLAG_LOWTAG which tells libata tag
allocation to do lowest-first.

However, with the recent switch to blk-mq tag allocation, the liata
tag allocation code path is no longer used and the workaround is now
implemented in the block layer and selected by setting
scsi_host_template->tag_alloc_policy to BLK_TAG_ALLOC_FIFO.  See
9269e23496dd ("libata: make sata_sil24 use fifo tag allocator").

This leaves ATA_FLAG_LOWTAG withoout any actual user.  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Shaohua Li <shli@fb.com>
Cc: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index b061ba2..fd9be17 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -4240,10 +4240,7 @@
 	unsigned int i, tag;
 
 	for (i = 0, tag = ap->sas_last_tag + 1; i < max_queue; i++, tag++) {
-		if (ap->flags & ATA_FLAG_LOWTAG)
-			tag = 1;
-		else
-			tag = tag < max_queue ? tag : 0;
+		tag = tag < max_queue ? tag : 0;
 
 		/* the last tag is reserved for internal command. */
 		if (tag == ATA_TAG_INTERNAL)