GCE Credentials should not derive from Scoped

[tseaver]

Because with_scopes raises NotImplementedError, the class cannot be used polymorphically where instances of Scoped are expected (see googleapis/google-cloud-python#3933). For instance, in C++,google.auth.compute_engine.credentials.Credentials wouldn't even be instantiable, because with_scopes is a pure virtual method cognate.

If there is a need for part of the interface to be expressed (e.g., the scopes, requires_scopes, and has_scopes methods), that should be factored into another class (maybed ReadonlyScoped?), with the current Scoped class extending it.