[SCSI] Remove unused method scsi_device_cancel

Removes an obsolete method scsi_device_cancel which isn't being used
anywhere in the kernel.

Signed-off-by: Priyanka Gupta <priyankag@google.com>
Acked-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>