You may remove the 2nd moulding or liner option from the customer's design page by setting the number of mouldings you want to offer to 1 under Menu -> Settings -> Customers. Please note that this will also hide the 3rd moulding or fillet.
Alternatively, the 2nd moulding or liner line can be hidden from the customer design page by adding the CSS below to Menu -> Settings -> Custom CSS Styling.
#moul2Simple {
display: none;
}