libata: remove SCT WRITE SAME support

This was already disabled a while ago because it caused I/O errors,
and it's severly getting into the way of the discard / write zeroes
rework.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>