Codechange: Deduplicate ResolveReal() for resolvers that don't use real sprite groups.

This may change behaviour when multiple loading/loaded stages are provided, as the various copies checked in different orders, however only one result is expected in these cases anyway.

src/newgrf_roadtype.h

@@ -40,8 +40,6 @@ struct RoadTypeResolverObject : public ResolverObject {
 		}
 	}
 
-	const SpriteGroup *ResolveReal(const RealSpriteGroup *group) const override;
-
 	GrfSpecFeature GetFeature() const override;
 	uint32 GetDebugID() const override;
 };