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This PR removes the `RatesController` to reduce redundant CryptoCompare
API calls in the multichain architecture.

The `RatesController` was introduced for fetching cryptocurrency
conversion rates (primarily for BTC and SOL) to support non-EVM chains.
However, this resulted in duplicate API calls to CryptoCompare since we
already have `MultichainAssetsRatesController` and
`TokenRatesController` handling rate fetching functionality. This
redundancy increased API usage unnecessarily.

- Removed `RatesController` initialization and configuration from the
Engine
- Deleted `RatesController` messenger setup and related controller
initialization files
- Removed multichain selectors that depended on `RatesController` state
(`selectMultichainConversionRate`, `selectMultichainCoinRates`)
- Cleaned up type definitions and state management to remove
`RatesController` references
- Updated test snapshots and initial background state

The existing `MultichainAssetsRatesController` and
`TokenRatesController` will continue to handle all rate-fetching needs
without the additional overhead of `RatesController`.

CHANGELOG entry: Removed redundant RatesController to optimize API usage

Fixes: <!-- Add issue number if applicable -->

```gherkin
Feature: Multichain rate fetching

  Scenario: User views portfolio balance with non-EVM accounts
    Given user has Bitcoin and Solana accounts configured
    And user is viewing their portfolio

    When the app fetches cryptocurrency rates
    Then rates should be fetched only through MultichainAssetsRatesController
    And no redundant API calls should be made to CryptoCompare
    And account balances should display with correct fiat conversions

  Scenario: User switches between EVM and non-EVM accounts
    Given user has both EVM and non-EVM accounts
    And user is viewing their wallet

    When user switches between different account types
    Then appropriate conversion rates should display for each account
    And rate updates should continue to work correctly
```

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

<!-- RatesController making redundant API calls alongside
MultichainAssetsRatesController -->

<!-- Single source of rate fetching through
MultichainAssetsRatesController, reducing API overhead -->

- [ ] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've completed the PR template to the best of my ability
- [ ] I've included tests if applicable
- [ ] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Removes RatesController across Engine, messengers, selectors, and
types; adds migration 106 to purge its persisted state; updates tests
and fixtures.
>
> - **Engine/Core**:
> - Remove `RatesController` initialization, context exposure, polling,
and state-change subscriptions in `Engine.ts` and `constants.ts`.
> - Drop `rates-controller-init` and its tests; delete
`rates-controller-messenger` and references in `messengers/index.ts`.
> - **Selectors**:
> - Remove `selectMultichainConversionRate` and
`selectMultichainCoinRates`; update `multichain.ts` and related tests
accordingly.
> - **Types**:
> - Strip `RatesController` types and entries from `types.ts`
(controllers, state, actions/events, controller names).
> - **Migration**:
> - Add migration `106` to remove `backgroundState.RatesController`;
register in `migrations/index.ts` with tests.
> - **Tests/Fixtures**:
> - Update snapshots and `initial-background-state.json` to omit
`RatesController`.
> - Adjust `Engine.test.ts` assertions to no longer expect
`RatesController`.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
da413b8. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->