remove unused methods in common_db_mixin
remove unused methods in common_db_mixin

The following methods are no longer used in CommonDbMixin:
- register_dict_extend_funcs
- _apply_filters_to_query
- _filter_non_model_columns

This patch removes them from neutron.

NeutronLibImpact

Change-Id: Ic7042cdcb29e95cc3a13292819d77abc3971fe8a

mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,openstack/neutron,mahak/neutron