block: remove unused variable 'def'

The 'def' local variable became unused after commit f382fb0b ("block: remove
legacy IO schedulers"), let's remove it.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hisao Tanabe <xtanabe@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>