blk-zoned: Remove the definition of blk_zone_start()

Commit e76239a3 ("block: add a report_zones method") removed the last
blk_zone_start() call. Hence also remove the definition of this function.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406200820.15180-1-bvanassche@acm.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>