blk-mq: remove duplicated definition of blk_mq_freeze_queue

As the prototype has been defined in "include/linux/blk-mq.h", the one
in "block/blk-mq.h" can be removed then.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>