Remove RefreshRuntime.findAffectedHostInstances (#30538)

I originally added this with a plan to visualize which nodes got updated
after a Fast Refresh. I didn't end up implementing that part, and to my
knowledge, no actively used integration actually does that or use this
method.

- [Webpack plugin doesn't use
it](https://github.com/pmmmwh/react-refresh-webpack-plugin/blob/f1c8b9a44198449093ca95f85af5df97925e1cfc/lib/runtime/RefreshUtils.js)
- [RN doesn't use
it](https://github.com/facebook/react-native/blob/23c5c42de01953ed7b8e8938c2d3a8fac82250a0/packages/react-native/Libraries/Core/setUpReactRefresh.js)
- [Global GitHub code
search](https://github.com/search?q=findAffectedHostInstances&type=code&p=1)
only shows copies of this code and the type definition based on it, but
not actual calls to it

We should be able to delete this without a problem.