[debugger] remove some dead code.

src/debug/mirrors.js

 // Copyright 2006-2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils) {
 "use strict";
 
 // ----------------------------------------------------------------------------
 // Imports
 
@@ skipping 1511 matching lines @@
 PropertyMirror.prototype.attributes = function() {
   return %DebugPropertyAttributesFromDetails(this.details_);
 };
 
 
 PropertyMirror.prototype.propertyType = function() {
   return %DebugPropertyTypeFromDetails(this.details_);
 };
 
 
-PropertyMirror.prototype.insertionIndex = function() {
-  return %DebugPropertyIndexFromDetails(this.details_);
-};
-
-
 /**
  * Returns whether this property has a getter defined through __defineGetter__.
  * @return {booolean} True if this property has a getter
  */
 PropertyMirror.prototype.hasGetter = function() {
   return this.getter_ ? true : false;
 };
 
 
 /**
@@ skipping 1496 matching lines @@
 // Functions needed by the debugger runtime.
 utils.InstallFunctions(utils, DONT_ENUM, [
   "ClearMirrorCache", ClearMirrorCache
 ]);
 
 // Export to debug.js
 utils.Export(function(to) {
   to.MirrorType = MirrorType;
 });
 })