remove model_query_scope from common db mixin
remove model_query_scope from common db mixin

The model_query_scope method is not used from common db mixin [1]. This
patch removes it on the path to eventually removing common db mixin
all together.

NeutronLibImpact

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

Change-Id: Ib8ce6e9f6000e9c01d28c1471b885b0f3c8a041b

openstack/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,mahak/neutron,huntxu/neutron,huntxu/neutron,noironetworks/neutron,noironetworks/neutron