chore: remove Singleton & replace getIApp with getApp (#5514)

src/providers/seventv/SeventvAPI.hpp

@@ -1,7 +1,5 @@
 #pragma once
 
-#include "common/Singleton.hpp"
-
 #include <functional>
 
 class QString;
@@ -11,7 +9,7 @@ namespace chatterino {
 
 class NetworkResult;
 
-class SeventvAPI : public Singleton
+class SeventvAPI final
 {
     using ErrorCallback = std::function<void(const NetworkResult &)>;
     template <typename... T>
@@ -19,7 +17,7 @@ class SeventvAPI : public Singleton
 
 public:
     SeventvAPI() = default;
-    ~SeventvAPI() override = default;
+    ~SeventvAPI() = default;
 
     SeventvAPI(const SeventvAPI &) = delete;
     SeventvAPI(SeventvAPI &&) = delete;