Remove usa_contact_id

This removes the usa_contact_id field from the Agency model. We don't need to store this in the database, only the YAML files. 

I don't know that there's a good way to test the removal of the field. All tests pass after removal. 

Because I felt like writing a test, I've added a test that saves the agency model and tests some of the custom bits of that. This is somewhat unrelated to the change made. 
I don't *think* so, because earlier in the migration path is the information for adding that field.