libata-sff: remove dead code

Ever since v2.6.19 the code contains a check for CONFIG_NO_ATA_LEGACY.
But that macro has never been defined. Apparently no one ran into
problems on platforms that do not support compatibility mode. So remove
this code that has been dead for over seven years.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tejun Heo <tj@kernel.org>