dillon1000/react

Delete Partial Renderer SSR implementation (#24868)

This removes the old server rendering implementation (the "Partial Renderer").
It was replaced in React 18 with a new streaming implementation (Fizz).

We hadn't removed it from the codebase yet because Facebook hadn't finished
rolling out Fizz in production; it's been behind a feature flag while we run
performance tests and migrate our internal infrastructure.

The diff to land Fizz will land imminently, and once it does, we can merge
this commit.