Remove adyen test helper
Remove adyen test helper

We don't use the test credentials anymore

StemboltHQ/solidus-adyen,StemboltHQ/solidus-adyen,freerunningtech/solidus-adyen,freerunningtech/solidus-adyen,StemboltHQ/solidus-adyen,StemboltHQ/solidus-adyen,freerunningtech/solidus-adyen,freerunningtech/solidus-adyen