Prevent validation issues with issuers fields without choices

[rvdsteege]

In pronamic/wp-pronamic-pay-mollie#53 (comment) it was suggested to remove iDEAL issuers from the https://github.com/pronamic/wp-pronamic-pay-mollie/ integration. And the same in:

• Actie vereist: Verwijder de keuze voor de kaartuitgever uit je checkout wp-pronamic-pay-mollie#62
• Remove iDEAL issuers code wp-pronamic-pay-mollie#63
• Actie vereist: Verwijder de keuze voor de kaartuitgever uit je checkout wp-mollie#7
• Remove iDEAL issuers code wp-mollie#8

However, if we suddenly remove the iDEAL issuers and forms are not updated yet, validation issues will occur with issuers fields marked as 'required' and users will be unable to submit the form:

To prevent these issues, we can remove issuers fields without choices from the form and mark them as valid during validation, as these are no longer needed.

[remcotolsma]

I don't know if it's smart for integrations like this to remove fields from forms. If there are references (in text or code) to this field in the form, will other problems arise? Falling back to https://github.com/pronamic/ideal-issuers might be a safer approach? With the official Mollie for WooCommerce plugin, there were also users who didn't like the fact that the iDEAL issuer field had simply been removed. For example, because the gateway text contained a reference to the iDEAL bank selection field.