[State Sync Redesign 3.9] Remove dual-write and `updateMetamaskState` thunk

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Remove the Redux dispatch path now that all components read from `ControllerStateManager`.

## Steps

1. Remove Redux dispatch from `requestAnimationFrame` batch flush in `ui/index.js`
2. Remove `updateMetamaskState` thunk from `ui/store/actions.ts`
3. Remove `UPDATE_METAMASK_STATE` action constant
4. Remove the `UPDATE_METAMASK_STATE` case from `ui/ducks/metamask/metamask.js`
5. Remove derived action dispatches (`CHAIN_CHANGED`, `ACCOUNT_CHANGED`, etc.)
6. Remove the `sendUpdate` notification handler from `ui/index.js` `onNotification`

## Files

- `ui/index.js`
- `ui/store/actions.ts`
- `ui/store/action-constants.ts`
- `ui/ducks/metamask/metamask.js`

## Dependencies

Issue 3.8 (all `forceUpdateMetamaskState` calls removed), all Issue 3.7 sub-issues complete

## Context

Part of [State Sync Redesign — Epic 3: UI-controller subscriptions].

Start with ui/index.js and verify the requestAnimationFrame batch flush and onNotification paths named in the issue. Then inspect ui/store/actions.ts, ui/store/action-constants.ts, and ui/ducks/metamask/metamask.js, after confirming Issue 3.8 and all Issue 3.7 sub-issues are complete. Done means the listed Redux and derived action paths, including sendUpdate, are removed without leaving UPDATE_METAMASK_STATE references.

Written by the indexing model from the issue text.