block, bfq: remove local variable 'bfqq_already_existing' in bfq_init_rq()

Now that 'bfqq_already_existing' is only used in one branch, it can be
removed. There are no functional changes.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240909134154.954924-7-yukuai1@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>