Remove unused variable

This fixes a compiler warning.

drivers/scsi/sd.c

@@ -756,7 +756,6 @@ static void
 sd_spinup_disk(struct scsi_disk *sdkp, char *diskname,
 	       struct scsi_request *SRpnt, unsigned char *buffer) {
 	unsigned char cmd[10];
-	struct scsi_device *sdp = sdkp->device;
 	unsigned long spintime_value = 0;
 	int the_result, retries, spintime;