libata: kill ATA_HORKAGE_DMA_RW_ONLY

ATA_HORKAGE_DMA_RW_ONLY for TORiSAN is verified to be subset of using
DMA for ATAPI commands which aren't aligned to 16 bytes.  As libata
now doesn't use DMA for unaligned ATAPI commands, the horkage is
redundant.  Kill it.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>