Remove DummyStatTimer since it's unused now

Since this isn't part of the API surface, removing it now that it's unused shouldn't cause any problems.

src/display/display_utils.js

@@ -16,7 +16,7 @@
 
 import {
   assert, BaseException, CMapCompressionType, isString, removeNullCharacters,
-  stringToBytes, unreachable, Util, warn
+  stringToBytes, Util, warn
 } from '../shared/util';
 
 const DEFAULT_LINK_REL = 'noopener noreferrer nofollow';
@@ -429,28 +429,6 @@ class StatTimer {
   }
 }
 
-/**
- * Helps avoid having to initialize {StatTimer} instances, e.g. one for every
- * page, in cases where the collected stats are not actually being used.
- * This (dummy) class can thus, since all its methods are `static`, be directly
- * shared between multiple call-sites without the need to be initialized first.
- *
- * NOTE: This must implement the same interface as {StatTimer}.
- */
-class DummyStatTimer {
-  constructor() {
-    unreachable('Cannot initialize DummyStatTimer.');
-  }
-
-  static time(name) {}
-
-  static timeEnd(name) {}
-
-  static toString() {
-    return '';
-  }
-}
-
 function isFetchSupported() {
   return (typeof fetch !== 'undefined' &&
           typeof Response !== 'undefined' && 'body' in Response.prototype &&
@@ -593,7 +571,6 @@ export {
   DOMCMapReaderFactory,
   DOMSVGFactory,
   StatTimer,
-  DummyStatTimer,
   isFetchSupported,
   isValidFetchUrl,
   loadScript,