Change and simplify algorithm for vehicle repair cost.

This is due to absurdly high repair costs for old vehicles with
old algorithm, due to implicit exponential increases.
Get rid of variable in vehicle struct.

src/vehicle_cmd.cpp

@@ -138,7 +138,6 @@ CommandCost CmdBuildVehicle(TileIndex tile, DoCommandFlag flags, uint32 p1, uint
 	if (value.Succeeded() && flags & DC_EXEC) {
 		v->unitnumber = unit_num;
 		v->value      = value.GetCost();
-		v->repair_cost = value.GetCost();
 
 		InvalidateWindowData(WC_VEHICLE_DEPOT, v->tile);
 		InvalidateWindowClassesData(GetWindowClassForVehicleType(type), 0);