null_blk: remove unused nullb device

The compiler rightfully complains:

drivers/block/null_blk_main.c: In function ‘null_complete_rq’:
drivers/block/null_blk_main.c:647:16: warning: unused variable ‘nullb’ [-Wunused-variable]
  struct nullb *nullb = rq->q->queuedata;
                ^~~~~

Fixes: 49f66136 ("nullb: remove leftover legacy request code")
Signed-off-by: Jens Axboe <axboe@kernel.dk>