Remove defaultProps support (except for classes) (#28733)

This removes defaultProps support for all component types except for
classes. We've chosen to continue supporting defaultProps for classes
because lots of older code relies on it, and unlike function components,
(which can use default params), there's no straightforward alternative.

By implication, it also removes support for setting defaultProps on
`React.lazy` wrapper. So this will not work:

```js
const MyClassComponent = React.lazy(() => import('./MyClassComponent'));
// MyClassComponent is not actually a class; it's a lazy wrapper. So
// defaultProps does not work.
MyClassComponent.defaultProps = { foo: 'bar' };
```

However, if you set the default props on the class itself, then it's
fine.

For classes, this change also moves where defaultProps are resolved.
Previously, defaultProps were resolved by the JSX runtime. This change
is only observable if you introspect a JSX element, which is relatively
rare but does happen.

In other words, previously `<ClassWithDefaultProp />.props.aDefaultProp`
would resolve to the default prop value, but now it does not.