Remove useless reference to RxJS definitions

This reference is not needed and is now causing problem because the
definitelyTyped for RxJS is now part of the RxJS package directly.  The
Nuget package dependency will be removed automatically.
Perhaps `toSubject(): Rx.ISubject<T>;` should be `toSubject(): Rx.Subject<T>;`. 

I don't know RX. Would appreciate @Igorbek 's review :)