fix: let wavesurfer use its default backend

forcefully setting it to WebAudio seemingly disable progress bar on my
Firefox Desktop for some reason
perhaps it actually disabled the fallback mechanism

src/js/player.js

@@ -114,7 +114,6 @@ page.reloadVideo = () => {
     if (isaudio) {
       options.plugins = {
         wavesurfer: {
-          backend: 'WebAudio',
           responsive: true
         }
       }