zbd: don't log "zone nnnn is not open" message

zbd: don't log "zone nnnn is not open" message

This log message has been added recently (it could have been my idea
to add it during internal review) and it turns out that the message
tends to flood the log when any decent workload is run with
--zonemode=zbd. Remove logging of this debug message.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>