pnfs-obj: No longer needed to take an extra ref at add_device

Andy's last device_cache patches, already take an extra
reference on the newly inserted device_id. So we can remove it
from obj-io.

Without this patch the device_ids are leaked.

Andy's patches are not in Linus tree yet. So I'm not sure if they are
scheduled for this Kernel or the next. This patch should be added as
part of these.

CC: Andy Adamson <andros@netapp.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>