fix: Exclude fetch polyfill from fetch plugin (#6838)

This fix excludes the use of
https://www.npmjs.com/package/fetch-polyfill because it does not support
a signal to cancel requests in progress.

lib/net/http_fetch_plugin.js

@@ -267,7 +267,8 @@ shaka.net.HttpFetchPlugin = class {
     } else {
       return false;
     }
-    return !!(window.fetch && window.AbortController);
+    return !!(window.fetch && !('polyfill' in window.fetch) &&
+      window.AbortController);
   }
 };