Remove (non-working) implementations of various commissioning/session APIs from MTRDeviceController base class. (project-chip#35781)

Also removes some API implementation bits from MTRDeviceController_Concrete,
since those are implemented in MTRDeviceController and delegate to APIs or
internal methods that subclasses should then implement.

Documents a few more things MTRDeviceController_XPC may need to implement at
some point.

Author: bzbarsky-apple