Remove polyfill script from Firefox, not needed

Author: fraz3alpha

browser-extensions/firefox/manifest.json

@@ -147,7 +147,6 @@
       "*://www.parkrun.co.nl/parkrunner/*/all/"
     ],
     "js": [
-      "js/lib/third-party/browser-polyfill/browser-polyfill-76eeeac.js",
       "js/lib/third-party/jquery/jquery-3.3.1.js",
       "js/lib/i18n.js",
       "js/content-scripts/content-script-parkrunner-summary.js"