revert(takeUntil): revert takeUntil implementation

per discussion with @staltz the takeUntil implementation shouldn't use
other operators (in this case .take()) to create other operators

src/Stream.ts

@@ -9,7 +9,6 @@ import {FoldOperator} from './operator/FoldOperator';
 import {LastOperator} from './operator/LastOperator';
 import {RememberOperator} from './operator/RememberOperator';
 import {StartWithOperator} from './operator/StartWithOperator';
-import {TakeUntilOperator} from './operator/TakeUntilOperator';
 import {
   CombineProducer,
   CombineInstanceSignature,
@@ -152,10 +151,6 @@ export class Stream<T> implements Observer<T> {
     return new Stream<T>(new StartWithOperator(this, x));
   }
 
-  takeUntil<T>(s: Stream<any>): Stream<T> {
-    return new Stream<T>(new TakeUntilOperator(this, s));
-  }
-
   merge(other: Stream<T>): Stream<T> {
     return Stream.merge(this, other);
   }