Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36106
Having `concurrentRoot` disabled when Fabric is enabled is not recommended.
This simplifies the setup and makes sure that both are either enabled or disabled.
## Changelog:
[iOS] [Breaking] - Hardcode concurrentRootEnabled to `true` when Fabric is enabled
Reviewed By: cortinico
Differential Revision: D43153402
fbshipit-source-id: d67aeb3413dbdf2430381aa44ede47ed00db32c6