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").

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