This argument was ignored since blk-mq was set as default, so remove it
from documentation.
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>

.txt file is now .rst
Signed-off-by: Jens Axboe <axboe@kernel.dk>