Remove redundant :house_id in House.set_attrs

Referring to the schema:
https://github.com/appacademy-demos/active_record_lite/blob/master/test/cats.sql

The class House doesn't have a 'house_id' column. Further, it belongs to the class Human, which already has that column, as a foreign key. It didn't break anything during the AR-Lite project, but it may still confuse current/future students.