purge polyfill from firefox branch; fix 'lookup doesn't work on certain strings when certain json dictionaries are loaded' bug

background.html

@@ -4,7 +4,6 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 </head>
 <body>
-<script src="include/browser-polyfill.min.js" charset="UTF-8"></script>
 <script src="background-script.js" charset="UTF-8"></script>
 <textarea id="nazeka-paste-target"></textarea>
 </body>