Delete cancelled billing agreement

You could also use `case` here:

```ruby
case ipn_msg[:type]
when *ORDER_UPDATE_TYPES
  PaypalService::PaypalPayment::Command.update(ipn_msg)
when :billing_agreement_cancelled
  PaypalService::PaypalAccount::Command.delete_cancelled_billing_agreement(ipn_msg[:payer_id], ipn_msg[:billing_agreement_id])
else
  raise NoMethodError
end
```

Looks cleaner imo