block, bfq: remove unused variable for bfq_queue

it defined in d0edc247

, but there's nowhere to use it,
so remove it.

Signed-off-by: Yuwei Guan <Yuwei.Guan@zeekrlife.com>
Acked-by: Paolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20221018030139.159-1-Yuwei.Guan@zeekrlife.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>