Fix/maksu 42 cancel action fix

[miikavai]

MAKSU-42

PROBLEM: When user cancels order and is returned back to cancel action, cancellation always fails. Order is not canceled and user is redirected to cart via error action.

EXPECTED RESULT: When user cancels order and is returned back to cancel action, cancellation is successful: order is cancelled and user is returned to shopping cart with proper error message

CHANGES INTRODUCED:

• Remove validation against pmt_reference (validateReturnedOrder) as cancel return call never contains that parameter causing validation always fail
• Add missing json_decode for payment additional data
• Redirect to cart instead of checkout for consistency and due to the fact that Magento hides error messages in checkout (https://github.com/magento/magento2/blob/2.3-develop/app/code/Magento/Checkout/view/frontend/layout/checkout_index_index.xml#L449)

[miikavai]

There seems to be also helper method used in the module to unserialize payment additional data, json_decode($payment->getAdditionalData(), true) could be changed to that: $this->getHelper()->getPaymentAdditionData($payment)