Do you mean that you can't do web design or that you don't want to?
If you don't want to then yes, I would phone them up and say that you have decided to withdraw your application.
If it is because you can't do it, surely they have seen your CV or the info that you have provided on the application form and shortlisted you for interview on that basis. They must know that you don't have experience in that area, perhaps they are willing to train the right person.