Added Apple pay by mistake.

We are not splitting hairs. We need to know exactly what you did to be able to help.

You just set up Apple Cash, but you don't actually have to use it if you don't want to.

If will just be there, but affects nothing and you have no obligation to use it.

You can of course just turn it off in Settings->Wallet & Apple Pay->Apple Cash, and turn it off.

Added where?

Remove from where?

There's no way to add Apple Pay. It is a payment mechanism that exists by default always whether you use it or not

You can add a card to Wallet to use Apple Pay, and as such you can remove the card too, by opening Wallet, tapping on the card, then on the (...) icon and then on Card Details, then on Remove Card.

If that is not what you did, please explain exactly what you did, and why you think you need to remove it.