chore: delete multichain accounts feature flag (#33802)

## **Description**

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Multichain accounts is permanently enabled, so the remote feature flag
helpers in `app/multichain-accounts/remote-feature-flag.ts` had been
reduced to stubs that always return `true`. Nothing in the codebase
imports `isMultichainAccountsRemoteFeatureEnabled` or
`isMultichainAccountsState2Enabled` anymore, so this PR deletes the
module and its test file to remove dead code.

## **Changelog**

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CHANGELOG entry: null

## **Related issues**

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Fixes: N/A

## **Manual testing steps**

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N/A — pure dead-code deletion with no runtime behavior change. Verified
via search that no remaining code references the deleted functions, and
CI (unit tests, TypeScript, lint) covers the rest.

## **Screenshots/Recordings**

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### **Before**

N/A

### **After**

N/A

## **Pre-merge author checklist**

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- [x] 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).
- [x] I've completed the PR template to the best of my ability
- [x] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] 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.

#### Performance checks (if applicable)

- [ ] I've tested on Android
  - Ideally on a mid-range device; emulator is acceptable
- [ ] I've tested with a power user scenario
- Use these [power-user
SRPs](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/edit-v2/401401446401?draftShareId=9d77e1e1-4bdc-4be1-9ebb-ccd916988d93)
to import wallets with many accounts and tokens
- [ ] I've instrumented key operations with Sentry traces for production
performance metrics
- See [`trace()`](/app/util/trace.ts) for usage and
[`addToken`](/app/components/Views/AddAsset/components/AddCustomToken/AddCustomToken.tsx#L274)
for an example

For performance guidelines and tooling, see the [Performance
Guide](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/400085549067/Performance+Guide+for+Engineers).

## **Pre-merge reviewer checklist**

- [ ] 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.

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Author: gantunesr