chore: remove settings manager from react-devtools-core (#30986)

Stacked on https://github.com/facebook/react/pull/30636. See [this
commit](https://github.com/facebook/react/pull/30986/commits/20cec76c44f77e74b3a85225fecab5a431cd986f).

This has been only used for React Native and will be replaced by another
approach (initialization via `installHook` call) in the next PR.