chore: replace usages of obsolete twitch struct with backing field (#3614)

Co-authored-by: Felanbird <41973452+Felanbird@users.noreply.github.com>

src/Application.hpp

@@ -58,18 +58,12 @@ public:
 
     CommandController *const commands{};
     NotificationController *const notifications{};
-    TwitchIrcServer *const twitch2{};
+    TwitchIrcServer *const twitch{};
     ChatterinoBadges *const chatterinoBadges{};
     FfzBadges *const ffzBadges{};
 
     /*[[deprecated]]*/ Logging *const logging{};
 
-    /// Provider-specific
-    struct {
-        /*[[deprecated("use twitch2 instead")]]*/ TwitchIrcServer *server{};
-        /*[[deprecated("use twitch2->pubsub instead")]]*/ PubSub *pubsub{};
-    } twitch;
-
 private:
     void addSingleton(Singleton *singleton);
     void initPubsub();