block: Remove request_queue.nr_queues

Commit 897bb0c7 ("blk-mq: Use proper cpumask iterator"; v4.6)
removed the last use of request_queue.nr_queues from outside
blk_mq_init_allocate_queue(). Remove this member variable to make
struct request_queue smaller. This patch does not change any
functionality.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>