Codechange: Use C++ features for train wagon overrides. (#9141)

This removes the need for C-style array management and allows use of iterators to perform wagon override lookups.

src/newgrf_engine.h

@@ -116,8 +116,6 @@ enum VehicleTrigger {
 };
 void TriggerVehicle(Vehicle *veh, VehicleTrigger trigger);
 
-void UnloadWagonOverrides(Engine *e);
-
 void AlterVehicleListOrder(EngineID engine, uint target);
 void CommitVehicleListOrderChanges();