Fifo: Get rid of undefined global

This declaration doesn't have a matching implementation
so it can be removed entirely.

Source/Core/VideoCommon/Fifo.h

@@ -4,7 +4,6 @@
 
 #pragma once
 
-#include <atomic>
 #include <cstddef>
 #include "Common/CommonTypes.h"
 
@@ -13,8 +12,6 @@ class PointerWrap;
 namespace Fifo
 {
 
-extern std::atomic<u8*> g_video_buffer_write_ptr_xthread;
-
 void Init();
 void Shutdown();
 void Prepare(); // Must be called from the CPU thread.