block: cleanup for _blk/blk_rq_prep_clone

Both cmd and sense had been moved to scsi_request, so remove
the related comments to avoid confusion.

And as Bart suggested, move _blk_rq_prep_clone into the only
caller (blk_rq_prep_clone).

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>