Remove unnecessary renewal order paid date update

[james-allan]

Fixes #791

Description

Back when we were working on WC 2.7 (which later became WC 3.0) we added some code to the WC_Subscriptions_Renewal_Order::maybe_record_subscription_payment() function which would set the renewal order's paid date to the current time.

Based on the original commit message (515c8e2) this was necessary because the paid date was set consistently as it required the gateways to call the payment_complete() function.

This is no longer necessary as WooCommerce core sets the paid date on status transition. See WC_Order::set_status()

Given the maybe_record_subscription_payment() function is hooked onto a status transition hook, it's really not necessary for us to set the paid date.

This PR:

1. Removes WC version compatibility code that is no longer needed (pre 3.0).
2. Removes the $order->set_date_paid( $current_time ) call when a renewal order status is changed.

Given we're no longer explicitly setting the paid date on a renewal order I did a review of the places where we use the paid date.

• There are some in the switch calculations where it gets the subscription's last order paid date.
  • This wouldn't be affected given the paid date is still set, it just won't be overridden if an order is reset back to unpaid and than paid.
• You can call $subscription->get_date( 'last_order_date_paid' ) and it will return the paid date of the most recent order, however I wasn't able to find any cases where we do that.

How to test this PR

1. Purchase a subscription.
2. From the edit subscription admin screen process a renewal from the actions dropdown.
3. Edit the order and make a note of the paid date.

^{Renewal order was paid @ 5:45 pm}

4. Mark the order as on-hold
   • On the issue this is described as the order was disputed.
5. Mark the order as processing or completed.
6. On trunk the renewal order paid date would be updated.

^{Renewal order was paid @ 5:50 pm}

8. On this branch the order paid date should not be updated.

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• Added deprecated functions, hooks or classes to the spreadsheet