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: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>