scsi: remove unused free discard page in sd_done

- sd_done isn't called for pc request so we never call the code.
- we use sd_unprep to free discard page now.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>