Restoring a Branded Tracking Link domain back to the Marketo default

Issue Description
You have a CNAME for your Branded Tracking Links in email but want to restore the settings to the system default (example:

Issue Resolution
Once you have set up a Branded Tracking Link domain, it is not possible to restore it to the Marketo default.  You can change it to a different CNAME domain, but it is not possible to revert to the original.