block_dump: remove comments in docs

Now block_dump feature is gone, remove all comments in docs.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210313030146.2882027-4-yi.zhang@huawei.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>