test: Remove feature flags from onboarding fixture (#37857)

## **Description**

Feature flags were accidentally added to the onboarding fixture
recently. This wasn't a problem for E2E tests because we refresh feature
flags as part of initialization (which itself was a bug), but when we
tried to fix this in #37552 it revealed this fixture problem.

The onboarding fixture has been updated to not include any feature
flags. Individual E2E tests can set the features they need using a
manifest override or a feature flag API mock.

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/37857?quickstart=1)

## **Changelog**

CHANGELOG entry: null

## **Related issues**

Unblocks #37552

## **Manual testing steps**

N/A

## **Screenshots/Recordings**

N/A

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/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-extension/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **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.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Strips `RemoteFeatureFlagController` data from
`test/e2e/fixtures/onboarding-fixture.json`, setting `cacheTimestamp` to
`0` and `remoteFeatureFlags` to `{}`.
> 
> - **E2E Fixture** (`test/e2e/fixtures/onboarding-fixture.json`):
> - Remove all `RemoteFeatureFlagController.remoteFeatureFlags` content.
>   - Set `RemoteFeatureFlagController.cacheTimestamp` to `0`.
>   - Keep other onboarding defaults unchanged.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
f70ce81. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Gudahtt