## Summary

Reject specialization of a subclass after all generic parameters in its
bases have been specialized, including by defaults.

```python
T = TypeVar("T")
DefaultT = TypeVar("DefaultT", default=str)

class Base(Generic[T, DefaultT]): ...
class Child(Base[int]): ...

Child[bytes]  # error: Child is not generic
```

The existing fallback treated any class with `Generic` anywhere in its
MRO as generic. That fallback predates our support for legacy generic
contexts and is now overly broad: `Generic` remains in the runtime MRO
even when the class has no free type variables. Actual generic class
literals are already accepted through their generic context, so we can
remove the MRO fallback and report `not-subscriptable` for other
classes.

This brings ty into conformance with the
`generics_defaults_specialization.py` case from
[python/typing#2325](python/typing#2325) and
adds focused mdtest coverage.