scsi: Simplify scsi_read()/_write()

With no non-DM driver using scsi_read()/_write() APIs, remove
the legacy implementations.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
[trini: Reorder slightly and mark scsi_read/write behind BLK test to
avoid warnings]
Signed-off-by: Tom Rini <trini@konsulko.com>