dcssblk: don't set bd_block_size in ->open

bd_block_size contains a value that matches the logic block size when
opening, so the statement is redundant.  Even if it wasn't the dumb
assignment would cause a a mismatch with bd_inode->i_blkbits.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>