blkdev.h: remove unused codes blk_account_rq

Last users of blk_account_rq gone with patch commit a1ce35fa
("block: remove dead elevator code") and now it gets no caller, it can
be safely removed.

Signed-off-by: Lin Feng <linf@wangsu.com>
Link: https://lore.kernel.org/r/20210512100124.173769-1-linf@wangsu.com


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