scsi: sd: Remove superfluous residual assignments

Since commit 26e85fcd ("[SCSI] sd: Permit merged discard requests";
kernel v3.10) sd_done() sets the residual not only for failed special
requests but also for special requests that succeeded. Hence remove the
code from functions called by sd_init_command() that sets the residual.
This patch does not change any functionality.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>