Twitch API: v5 to Helix migration (#1560)

There's a document in src/providers/twitch/api which describes how we interact with the Twitch API.
Keeping this up to date might be a healthy way for us to ensure we keep using the right APIs for the right job.

src/singletons/Toasts.hpp

@@ -29,9 +29,5 @@ private:
 #ifdef Q_OS_WIN
     void sendWindowsNotification(const QString &channelName, Platform p);
 #endif
-
-    static void fetchChannelAvatar(
-        const QString channelName,
-        std::function<void(QString)> successCallback);
 };
 }  // namespace chatterino