Remove Reconciler fork (2/3)

[kassens]

We've heard from multiple contributors that the Reconciler forking mechanism was confusing and/or annoying to deal with. Since it's currently unused and there's no immediate plans to start using it again, this removes the forking.

Fully removing the fork is split into multiple PRs to better preserver history.

1. Remove Reconciler fork (1/2) #25774 Remove the forking infra:
   • remove enableNewReconciler feature flag.
   • remove unstable_isNewReconciler export
   • remove eslint rules for cross fork imports
   • remove *.new.js files and update imports
2. Remove Reconciler fork (2/3) #25776 (this PR) Merge non-suffixed files into *.old files where both exist
3. Remove Reconciler fork (2/2) #25775 Rename *.old files

[kassens]

merged into #25774