[Fiber/DevTools] Stop injecting findHostInstanceByFiber (#30519)

This is not used by DevTools since it has its own implementation of it.

This function is getting removed since `findDOMNode` is getting removed
so we shouldn't keep around extra bytes unnecessarily.

There is also `findHostInstancesForRefresh` which should really be
implemented on the `react-refresh` side. Not using an injection but
that's a heavier lift and only affects `__DEV__`.