libata: kill unused ata_flush_cache()

ata_flush_code() hasn't been in use for quite some time now.  Kill it.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Jeff Garzik <jeff@garzik.org>