LU-6856 zfs: handle non existing file in osd_object_ref_del

LU-6856 zfs: handle non existing file in osd_object_ref_del

Remove false assertion in zfs:osd_object_ref_del() because this
may be in the cleanning path of error handling.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ib7b9d80816bdab7f68b36a33e95140ea7f3eae8c
Reviewed-on: http://review.whamcloud.com/15611
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>