Remove COURSEWARE_ENABLED flag

Per Dave Ormsbee:
> That was from way back when, when we actually had Django server pools,
> proxied differently by nginx; one was the courseware, and the other
> was askbot, a discussion forum.
>
> They shared all the same code, but different parts were live. It was
> about as good an idea as it sounds.
>
> There is no reason that I can think of in the current day and age where
> you would ever want to run the LMS and *turn off courseware*.