Core: Remove UpdateFPSDisplay

This is effectively unused, as the window handles that we pass to the
GLInterface are window handles for the frame which isn't ever a real
toplevel window. Host_UpdateTitle is what actually sets the proper title
on the render window.

Source/Core/VideoCommon/VideoBackendBase.h

@@ -68,8 +68,6 @@ public:
 
 	virtual void EmuStateChange(EMUSTATE_CHANGE) = 0;
 
-	virtual void UpdateFPSDisplay(const std::string&) = 0;
-
 	virtual unsigned int PeekMessages() = 0;
 
 	virtual bool Initialize(void *window_handle) = 0;