[Fiber] Delete isMounted internals (#31966)

The public API has been deleted a long time ago so this should be unused
unless it's used by hacks. It should be replaced with an
effect/lifecycle that manually tracks this if you need it.

The problem with this API is how the timing implemented because it
requires Placement/Hydration flags to be cleared too early. In fact,
that's why we also have a separate PlacementDEV flag that works
differently.


https://github.com/facebook/react/blob/main/packages/react-reconciler/src/ReactFiberCommitWork.js#L2157-L2165

We should be able to remove this code now.