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,
72dd299d50 ("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
9269e23496 ("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>

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 */