block/rnbd: Kill destroy_device_cb

We can use destroy_device directly since destroy_device_cb is just the
wrapper of destroy_device.

Signed-off-by: Guoqing Jiang <guoqing.jiang@ionos.com>
Reviewed-by: Danil Kipnis <danil.kipnis@ionos.com>
Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210419073722.15351-8-gi-oh.kim@ionos.com


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