Remove other double import (#5565)

Debouncer is no longer used in the file, even as a type name.

Author: rictic

lib/utils/flush.js

@@ -8,9 +8,6 @@ Code distributed by Google as part of the polymer project is also
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 */
 import './boot.js';
-/* eslint-disable no-unused-vars */
-import { Debouncer } from '../utils/debounce.js';  // used in type annotations
-/* eslint-enable no-unused-vars */
 import {enqueueDebouncer, flushDebouncers} from '../utils/debounce.js';
 export {enqueueDebouncer};