scsi: xen-scsifront: Remove code that zeroes driver-private command data

Since the SCSI core zeroes driver-private command data, remove
that code from the xen-scsifront driver.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: xen-devel@lists.xenproject.org
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>