Removing unused method: _remaining_items

Removing method _remaining_items from object controller class.

The only caller to this function was removed as part of the
work to move all DLO functionality to middleware:
https://review.openstack.org/63326

Change-Id: I7fbc208746bba8142ae51bf27cfa1979cae00301
Signed-off-by: Thiago da Silva <thiago@redhat.com>