Remove deprecated/legacy `PaymentInfo` properties

[remcotolsma]

I have cleaned up almost all deprecated/legacy properties and functions: https://github.com/pronamic/wp-pay-core/compare/ba92e39b4f12b31b9a8a696130b6dfbc4fcce510...f8f12c359b14881a74f866015b5e8e85a453862f

A few properties still require some work: https://github.com/pronamic/wp-pay-core/blob/develop/src/Payments/PaymentInfo.php#L31-L35

 * @property string|null $email
 * @property string|null $customer_name
 * @property int|string|null $user_id
 * @property mixed $user_agent
 * @property mixed $user_ip

• [x] $email
• [x] $customer_name
• [x] $user_id
• [x] $user_agent
• [x] $user_ip

Also see: https://github.com/pronamic/wp-pronamic-pay/issues/156.