Qt: Remove orpahned function declaration

src/citra_qt/main.h

@@ -73,12 +73,6 @@ private:
 
     void ConnectWidgetEvents();
 
-    /**
-     * Initializes the emulation system.
-     * @param system_mode The system mode with which to intialize the kernel.
-     * @returns Whether the system was properly initialized.
-     */
-    bool InitializeSystem(u32 system_mode);
     bool LoadROM(const QString& filename);
     void BootGame(const QString& filename);
     void ShutdownGame();