Remove useless reference to RxJS

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 will needed to be changed also.

knockout.rx/knockout.rx.d.ts

@@ -4,7 +4,6 @@
 // Definitions: https://github.com/borisyankov/DefinitelyTyped
 
 /// <reference path="../knockout/knockout.d.ts"/>
-/// <reference path="../rx.js/rx.d.ts"/>
 
 interface KnockoutSubscribableFunctions<T> {
 	toObservable(event?: string): Rx.Observable<T>;