VideoCommon/Statistics: Remove global system accessor from s_after_frame_event

Instead, we make the event take a reference to the system and then pass
it in when the event is triggered.

This does introduce two other accessors, but these are much easier to
refactor out over time, and without modification to the existing event
interface.

Source/Core/VideoCommon/FramebufferManager.cpp

@@ -84,7 +84,8 @@ bool FramebufferManager::Initialize()
     return false;
   }
 
-  m_end_of_frame_event = AfterFrameEvent::Register([this] { EndOfFrame(); }, "FramebufferManager");
+  m_end_of_frame_event =
+      AfterFrameEvent::Register([this](Core::System&) { EndOfFrame(); }, "FramebufferManager");
 
   return true;
 }