Remove react-test-renderer/shallow export (#28497)

Based on
- https://github.com/facebook/react/pull/28419

## Summary

The shallow renderer was extracted from the repo years ago and published
by enzyme: https://github.com/enzymejs/react-shallow-renderer

We no longer need to reexport under the react-test-renderer namespace.
People can import `react-shallow-renderer` as needed

## How did you test this change?

- Observe shallow.js in react-test-renderer package from standard build
- Run build with changes on this branch
- Observe no more shallow.js export in build output