Remove owned_* methods.

Users who want this functionality can do so by subclassing tf.Module and
implemeting the methods themselves using `_flatten`. For example:

```
class MyModule(tf.Module):
  @property
  def owned_variables(self):
    is_variable = lambda v: isinstance(v, tf.Variable)
    return tuple(self._flatten(recursive=False, predicate=is_variable))
```

PiperOrigin-RevId: 232523894