brd: remove max_hw_sectors queue limit

Now we depend on blk_queue_split() to respect most of queue limit
(the only one exception could be dma alignment), however
blk_queue_split() isn't used for brd, so this limit isn't respected
since v4.3.

Also max_hw_sectors limit doesn't play a big role for brd, which is
added since brd is added to tree for unknown reason.

So remove it.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>