Delete stale _mollie_order_id meta on renewal order creation

[fjbender]

Summary

Fixes #1250 — a regression introduced in 8.1.8 silently blocks the "Pay for order" retry link for failed WooCommerce Subscriptions renewals whenever the subscription's initial payment went through the Orders API.

delete_renewal_meta() already strips _mollie_payment_id and _mollie_cancelled_payment_id that WooCommerce Subscriptions copies from the parent order onto a new renewal order, but it never cleared _mollie_order_id. Renewal orders always pay via MolliePayment (Payments API) and never create a Mollie Order themselves, so any _mollie_order_id present on a renewal order is always stale data inherited from the parent order.

cancelExistingMolliePaymentIfPending() (added in 8.1.8) checks _mollie_order_id before _mollie_payment_id, so it was resolving that stale, non-cancelable parent Order resource instead of the renewal's own failed SEPA Direct Debit payment — leading to a silent ['result' => 'failure'] on manual retry.

This is a one-line fix at the root cause: strip _mollie_order_id alongside the other inherited meta at renewal-order creation time, so it's never present to be misread later.

Test plan

• Subscription with initial payment via Orders API (e.g. iDEAL) + SEPA Direct Debit renewal
• Force a renewal payment failure, confirm the renewal order has no _mollie_order_id meta after creation
• Use the "Pay for order" link and confirm the customer is redirected to a new Mollie payment instead of silently failing
• Confirm initialPaymentUsedOrderAPI() behavior is unaffected (it reads the meta off the parent order, not the renewal order)