remove safe_reference from common db mixin

The safe_reference property in common db mixin is only used by
vmware-nsx and they can use weakref directly (see depends on patch).
This patch removes safe_reference as we're working towards removing
common db mixin all together.

Depends-On: https://review.openstack.org/#/c/571221/

[1] http://codesearch.openstack.org/?q=%5C.safe_reference

Change-Id: I77f9561139fd88eff51638f6f41cf79f0b876dc1

neutron/db/common_db_mixin.py

@@ -13,8 +13,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import weakref
-
 from neutron.db import _model_query
 from neutron.db import _resource_extend
 from neutron.db import _utils as ndb_utils
@@ -35,11 +33,6 @@ class CommonDbMixin(object):
     def register_dict_extend_funcs(resource, funcs):
         _resource_extend.register_funcs(resource, funcs)
 
-    @property
-    # TODO(HenryG): Remove; used only by vmware-nsx.
-    def safe_reference(self):
-        return weakref.proxy(self)
-
     @staticmethod
     def model_query_scope(context, model):
         return ndb_utils.model_query_scope_is_project(context, model)