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,
72dd299d ("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
9269e234 ("libata: make sata_sil24 use fifo tag allocator").

This leaves ATA_FLAG_LOWTAG withoout any actual user.  Remove it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Shaohua Li <shli@fb.com>
Cc: Dan Williams <dan.j.williams@intel.com>

drivers/ata/libata-scsi.c

@@ -4240,9 +4240,6 @@ int ata_sas_allocate_tag(struct ata_port *ap)
 	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;
 
 		/* the last tag is reserved for internal command. */

drivers/ata/sata_sil24.c

@@ -246,7 +246,7 @@ enum {
 	/* host flags */
 	SIL24_COMMON_FLAGS	= ATA_FLAG_SATA | ATA_FLAG_PIO_DMA |
 				  ATA_FLAG_NCQ | ATA_FLAG_ACPI_SATA |
-				  ATA_FLAG_AN | ATA_FLAG_PMP | ATA_FLAG_LOWTAG,
+				  ATA_FLAG_AN | ATA_FLAG_PMP,
 	SIL24_FLAG_PCIX_IRQ_WOC	= (1 << 24), /* IRQ loss errata on PCI-X */
 
 	IRQ_STAT_4PORTS		= 0xf,

include/linux/libata.h

@@ -231,7 +231,6 @@ enum {
 	ATA_FLAG_SW_ACTIVITY	= (1 << 22), /* driver supports sw activity
 					      * led */
 	ATA_FLAG_NO_DIPM	= (1 << 23), /* host not happy with DIPM */
-	ATA_FLAG_LOWTAG		= (1 << 24), /* host wants lowest available tag */
 
 	/* bits 24:31 of ap->flags are reserved for LLD specific flags */