Fix video not loading on iOS

Looks like Twitch made some updates to their mobile player on iOS where
it will first navigate to a bunch of URLs (probably related to ads
or analytics based on the domains). This resulted in the
`NavigationDelegate` not picking this up and preventing the loading by
default.

To fix, I've just removed the `NavigationDelegate` because it's
legacy code that really isn't needed anymore.

lib/screens/channel/video/video_store.dart

@@ -54,13 +54,6 @@ abstract class VideoStoreBase with Store {
         ..setNavigationDelegate(
           NavigationDelegate(
             onPageFinished: (_) => initVideo(),
-            // Used for preventing accidental navigation in the webview.
-            onNavigationRequest: (request) {
-              if (request.url.startsWith('https://player.twitch.tv')) {
-                return NavigationDecision.navigate;
-              }
-              return NavigationDecision.prevent;
-            },
           ),
         )
         ..loadRequest(Uri.parse(videoUrl));