Payment block: For one-off payments, remove requirement of WP.com account

### What

Users are prompted to create a WPcom account even if they are paying for something as a one-off ( Say someone purchasing a book on a simple site ) . This is unnecessary since the use case here does not involve recurring payments, nor member access. The use case is similar to that of a Paypal block.

It would be good to have an option where if Frequency is One off, have a toggle to disable requirement of WP.com account.

### Why

This will help users sell products to users and save dropouts for customers that are wary of creating a WP.com account.

### How

Related chat:
#33412959-hc

Start by locating the Payment block and the handling for the One off frequency, then trace where the WP.com account requirement is enforced. Done means one-off payments can disable that requirement while recurring payments and member-access flows retain it; add or update coverage for both paths if the existing test entry point is found.

Written by the indexing model from the issue text.