Yes, you need to inform them by calling. UA will set up a ticket in the infant's name that's attached to your revenue ticket. For domestic flights it is free, but helps them ensure you end up seated in the right places (where there are enough oxygen masks, etc.) and there are sufficient infant life vests.

While I have never not used a lap infant ticket, I would imagine you could just not use as any other ticket. You will likely have an easier time not using a ticket that is set up than setting one up at the last minute.