[nostalgia/core] Remove SignalHandler as parent to GBA version of AssetRef

src/nostalgia/core/assetmanager.hpp

@@ -263,7 +263,7 @@ class AssetManager {
 };
 #else
 template<typename T>
-class AssetRef: public ox::SignalHandler {
+class AssetRef {
 	private:
 		T &m_obj;