Codechange: use "[[maybe_unused]]" instead of a wide variety of other ways we had

While at it, replace OTTD_ASSERT with WITH_ASSERT, as this
is always set if assert() is valid. No matter if NDEBUG is set
or not.

src/vehicle.cpp

@@ -953,9 +953,7 @@ void CallVehicleTicks()
 	PerformanceAccumulator::Reset(PFE_GL_AIRCRAFT);
 
 	for (Vehicle *v : Vehicle::Iterate()) {
-#ifdef WITH_ASSERT
-		size_t vehicle_index = v->index;
-#endif
+		[[maybe_unused]] size_t vehicle_index = v->index;
 
 		/* Vehicle could be deleted in this tick */
 		if (!v->Tick()) {