Finish deleting Mutable from router implementation (#88046)
Based on:
- #87991
---
The Mutable type was used by an earlier implementation of App Router
when the reducer used to run during React's render phase. It was used to
prevent certain operations from running multiple times if an update was
reapplied by React.
Now that router "reducer" actions run outside React's render phase, we
no longer need this indirection — we can compute the next state object
directly.
Most of the Mutable-related logic was deleted in previous PRs; this
finishes the migration by inlining handleMutable into its callers.
