Core/Core: Remove another leftover part of frameskip

Source/Core/Core/Core.h

@@ -66,7 +66,6 @@ void DisplayMessage(const std::string& message, int time_in_ms);
 
 void FrameUpdateOnCPUThread();
 
-bool ShouldSkipFrame(int skipped);
 void VideoThrottle();
 void RequestRefreshInfo();