Remove `mutable` field from action types (#59221)
Similar in spirit to #58938.
The app router reducer state used to be managed by useReducer, so it was
written to be resilient to rebasing — the same action may be processed
multiple times. Now that we've lifted the reducer outside of React
(#56497), each action runs only a single time. So we can simplify some
of the logic.
The purpose of the `mutable` field was so that if an action is processed
multiple times, state could be reused between each run; for example, to
prevent redundant network fetches. Now that this scenario can no longer
happen, we can remove it.
I had to update some of the unit tests in navigate-reducer because they
were written with the assumption that the reducer was called multiple
times. As far as I can tell, most of this behavior is covered by e2e
tests anyway, so I don't think it's too risky.
Closes NEXT-1782
