Discord Rich Presence CMake integration

I have no idea if this works or not. Hopefully the build bot will tell me.

Source/Core/UICommon/DiscordPresence.h

@@ -4,8 +4,6 @@
 
 #pragma once
 
-#include "Common/Config/Config.h"
-
 namespace Discord
 {
 void Init();
@@ -13,5 +11,3 @@ void UpdateDiscordPresence();
 void Shutdown();
 void SetDiscordPresenceEnabled(bool enabled);
 }  // namespace Discord
-
-extern const Config::ConfigInfo<bool> MAIN_USE_DISCORD_PRESENCE;