[State Sync Redesign 2.4] Replace `this.memStore` with direct messenger subscriptions

After Issues 2.1, 2.2, 2.3, and Epic 1 Issue 1.3, `this.memStore` has no remaining consumers. Remove it.

## Steps

1. Verify no remaining `memStore` consumers (all migrated in Issues 2.1-2.3 and Epic 1)
2. Remove `this.memStore` construction and the ~68-line controller list in `updateStructure`
3. Remove `ComposableObservableStore` import (if `this.store` also removed, see Issue 2.5)

## Files

- `app/scripts/metamask-controller.js`

## Risk

Low — all consumers already migrated.

## Dependencies

Issues 2.1, 2.2, 2.3, and Epic 1 Issue 1.3

## Context

Part of [State Sync Redesign — Epic 2: Background cleanup].

Start in app/scripts/metamask-controller.js by verifying that Issues 2.1–2.3 and Epic 1 Issue 1.3 left no memStore consumers. Remove the this.memStore construction and the approximately 68-line controller list in updateStructure, then remove the ComposableObservableStore import if it is no longer needed; check Issue 2.5 if this.store is also removed.

Written by the indexing model from the issue text.