blk-mq: remove unused 'nr_expired' from blk_mq_hw_ctx

There is no usage of 'nr_expired'.

The 'nr_expired' was introduced by commit 1d9bd516 ("blk-mq: replace
timeout synchronization with a RCU and generation based scheme"). Its usage
was removed since commit 12f5b931 ("blk-mq: Remove generation
seqeunce").
Signed-off-by: NDongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/linux/blk-mq.h

@@ -57,7 +57,6 @@ struct blk_mq_hw_ctx {
 	unsigned int		queue_num;
 
 	atomic_t		nr_active;
-	unsigned int		nr_expired;
 
 	struct hlist_node	cpuhp_dead;
 	struct kobject		kobj;