RBAC's BindingMapping currently handles two cases of old representations:

• A single group can be referenced multiple times.
• The bound principals can be stored as a list instead of a dict/JSON object. (The keys in that object are called "source keys").

This ticket is only concerned with the second.

Per discussion on Slack, the old principals list representation no longer appears in actual databases. Verify that this is true, then remove the code and test cases that handle this representation. Ideally, also add checks to ensure that new code does not attempt to create these representations.