[`MetamaskController` A.8] Remove `as never` cast from `register.ts`

**Sub-epic:** [#41736 — `MetamaskController` decomposition: wallet-services completion](https://github.com/MetaMask/metamask-extension/issues/41736)
**File:** `app/scripts/lib/wallet-services/register.ts` (created by A.1–A.6, retyped in A.7)
**Size:** S

## Problem

After A.7 composes `WalletServicesActions` and retypes `registerWalletServices`, the `as never` cast in `register.ts` (introduced during A.1–A.6 to shim the type mismatch between `RootMessenger`'s default generic and each module's structural messenger type) becomes unnecessary.

## Solution

Remove the cast and confirm `tsc` passes.

## Acceptance criteria

- [ ] No `as never` (or any other cast) in `register.ts`
- [ ] `tsc` passes

## Dependencies

**A.7** — union must be composed first.

Open app/scripts/lib/wallet-services/register.ts and review the registerWalletServices typing after dependency A.7's WalletServicesActions union was composed. Remove the remaining cast without adding another cast, then run tsc. Done means register.ts contains no casts and TypeScript compilation passes.

Written by the indexing model from the issue text.