Comment 6 for bug 1434687

So the messy part about removing vol_get_usage_by_time is that there are db api tests for volume_usages that are using it to get information out of the database to verify that vol_usage_update is working correctly.

It seems when https://review.openstack.org/#/c/84254/ happened it cut a bit too deep, else we could drop the methods from conductor api/rpcapi since they won't work w/o the method in the conductor manager, but we could leave the DB APIs in for testing purposes.