null_blk: remove set but not used variable 'q'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/block/null_blk_main.c: In function 'end_cmd':
drivers/block/null_blk_main.c:609:24: warning:
 variable 'q' set but not used [-Wunused-but-set-variable]

It not used any more after commit
e50b1e32

 ("null_blk: remove legacy IO path")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>