DolphinQT/Host: Remove unimplemented prototypes

Remnants of prior existing code.

Source/Core/DolphinQt/Host.h

@@ -21,9 +21,6 @@ public:
 
   static Host* GetInstance();
 
-  void DeclareAsHostThread();
-  bool IsHostThread();
-
   bool GetRenderFocus();
   bool GetRenderFullFocus();
   bool GetRenderFullscreen();