civicrm/payment/ipn url unnecessarily requires mode

The recommended ipn url for payment processors is

civicrm/payment/ipn?processor_id=2

Where the processor is a test processor the url requires mode=test.

ie

civicrm/payment/ipn?processor_id=2&mode=test

This makes sense when we use the deprecated

civicrm/payment/ipn?processor_name=paypal&mode=test

but is redundant when ID is passed.

It causes problems when the payment processor chokes on the ampersand or url_encodes it. The simplest solution is to remove the redundant bit of the url