block: remove a pointless self-reference in block_dev.c

There is no point in duplicating the file name in the top of the file
comment.

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