Fix/cdodge/courseware index perf tweek

@cpennington @ormsbee 

It seems like were doing some unnecessary roundtrips to MongoDB (MongoHQ) in the index() function of courseware, which gets a lot of traffic in prod.

We already have the descriptor w/ children (just a few lines up), so we don't need to call get_module() which refetches the descriptor and its children. This saves about 3 or 4 round trips to MongoDB per page load on the index() view.