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/VideoBackends/OGL/VideoBackend.h

@@ -19,7 +19,6 @@ class VideoBackend : public VideoBackendHardware
 
 	void ShowConfig(void* parent) override;
 
-	void UpdateFPSDisplay(const std::string&) override;
 	unsigned int PeekMessages() override;
 };