[PATCH] libata: Remove obsolete flag

ATA_FLAG_IRQ_MASK was added when I did the original data transfer with
IRQ masked bits for PIO. It has since been replaced by ->pio_data_xfer
methods so should be removed so nobody uses it by mistake thinking it
still works.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>