block: Remove bdev_nr_zones()

Remove this function since it has no callers. This function was
introduced in commit 6cc77e9c ("block: introduce zoned block
devices zone write locking").

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Matias Bjorling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>