bfq: Remove useless code in bfq_lookup_next_entity

It is no need to judge entity is null or not here,
directly return entity is ok, so remove it.

Signed-off-by: GuoYong Zheng <zhenggy@chinatelecom.cn>
Link: https://lore.kernel.org/r/1655461684-19075-1-git-send-email-zhenggy@chinatelecom.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>