blk-mq: remove RQF_MQ_INFLIGHT

Since the previous patch change to only account active requests when
we really allocate the driver tag, the RQF_MQ_INFLIGHT can be removed
and no double account problem.

1. none elevator: flush request will use the first pending request's
   driver tag, won't double account.

2. other elevator: flush request will be accounted when allocate driver
   tag when issue, and will be unaccounted when it put the driver tag.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230913151616.3164338-3-chengming.zhou@linux.dev


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