scsi: Remove Scsi_Host.uspace_req_q

A patch I wrote myself several years ago removed SCSI target support
from the code under drivers/scsi. That patch removed the code that sets
uspace_req_q to a non-NULL value. Hence also remove the code that
depends on uspace_req_q != NULL.

References: commit 06646525 ("tgt: removal")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>