rp: remove _HasAResourceProvider mixin
rp: remove _HasAResourceProvider mixin

The _HasAResourceProvider mixin is no longer needed since the results
of AllocationList.get_all_by_XXX() and InventoryList.get_all_by_XXX()
have been modified to manually construct the Allocation and Inventory
POD objects using optimized non-ORM queries, so removing the mixin.

blueprint: de-orm-resource-providers

Change-Id: I1cc50c2e2501fef1fe8bad8451f833380371def8

phenoxim/nova,mikalstill/nova,openstack/nova,rahulunair/nova,openstack/nova,klmitch/nova,gooddata/openstack-nova,klmitch/nova,gooddata/openstack-nova,mikalstill/nova,gooddata/openstack-nova,mikalstill/nova,phenoxim/nova,klmitch/nova,klmitch/nova,mahak/nova,gooddata/openstack-nova,rahulunair/nova,mahak/nova,openstack/nova,rahulunair/nova,mahak/nova