Codechange: "set but not used" warning when disabling assert() (#9613)

DebugCheckSanity() is unused when asserts are disabled. While at it,
use WITH_ASSERT over NDEBUG, as that means we also run this code
during beta/RC.

src/order_cmd.cpp

@@ -609,6 +609,7 @@ bool OrderList::IsCompleteTimetable() const
 	return true;
 }
 
+#ifdef WITH_ASSERT
 /**
  * Checks for internal consistency of order list. Triggers assertion if something is wrong.
  */
@@ -642,6 +643,7 @@ void OrderList::DebugCheckSanity() const
 			(uint)this->num_orders, (uint)this->num_manual_orders,
 			this->num_vehicles, this->timetable_duration, this->total_duration);
 }
+#endif
 
 /**
  * Checks whether the order goes to a station or not, i.e. whether the
@@ -1782,7 +1784,7 @@ void CheckOrders(const Vehicle *v)
 		/* Do we only have 1 station in our order list? */
 		if (n_st < 2 && message == INVALID_STRING_ID) message = STR_NEWS_VEHICLE_HAS_TOO_FEW_ORDERS;
 
-#ifndef NDEBUG
+#ifdef WITH_ASSERT
 		if (v->orders.list != nullptr) v->orders.list->DebugCheckSanity();
 #endif