Revert of Introduce ExclusiveStreamReader.

Source/core/streams/ReadableStream.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 enum ReadableStreamState {
     "readable",
     "waiting",
     "closed",
     "errored"
 };
 
 [
     RuntimeEnabled=ExperimentalStream,
     GarbageCollected,
     Exposed=(Window,Worker),
     ActiveDOMObject,
 ] interface ReadableStream {
     [CallWith=ScriptState, RaisesException] any read();
     [CallWith=ScriptState] readonly attribute Promise ready;
     [ImplementedAs=stateString] readonly attribute ReadableStreamState state;
 
     [CallWith=ScriptState] Promise cancel(any reason);
-    [RaisesException] ExclusiveStreamReader getReader();
 
     [CallWith=ScriptState] readonly attribute Promise closed;
 };