chore: Remove obsolete preview build support (#27968)

## **Description**

We had previously configured CI to support installing `@metamask`-
scoped packages from the GitHub npm registry, as we used this registry
for "preview builds" from the core repository in the past. This is no
longer used; we publish preview builds to npm now instead. The obsolete
CI changes have been removed.

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/27968?quickstart=1)

## **Related issues**

* [Preview build
instructions](https://github.com/MetaMask/core/blob/main/docs/contributing.md#testing-changes-to-packages-with-preview-builds)
([permalink](https://github.com/MetaMask/core/blob/56efd1d13a8873a5abb9bd9880d0576148b9d1e4/docs/contributing.md#testing-changes-to-packages-with-preview-builds))
* Preview build related extension PRs: #16547, #19970, #20096, #20312


## **Manual testing steps**

This is not anticipated to have any functional impact. If it does break
something, it would be the install step, so we can test this by
installing locally (and seeing that the install step succeeded on CI).

## **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/develop/.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/develop/.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.

.circleci/config.yml

@@ -444,7 +444,7 @@ jobs:
       - gh/install
       - run:
           name: Install dependencies
-          command: .circleci/scripts/install-dependencies.sh
+          command: yarn --immutable
       - save_cache:
           key: dependency-cache-{{ checksum "/tmp/YARN_VERSION" }}-{{ checksum "yarn.lock" }}
           paths:

.yarnrc.yml

@@ -119,14 +119,6 @@ npmAuditIgnoreAdvisories:
   - 'react-beautiful-dnd (deprecation)'
   # New package name format for new versions: @ethereumjs/wallet.
   - 'ethereumjs-wallet (deprecation)'
-npmRegistries:
-  'https://npm.pkg.github.com':
-    npmAlwaysAuth: true
-    npmAuthToken: '${GITHUB_PACKAGE_READ_TOKEN-}'
-
-npmScopes:
-  metamask:
-    npmRegistryServer: '${METAMASK_NPM_REGISTRY:-https://registry.yarnpkg.com}'
 
 plugins:
   - path: .yarn/plugins/@yarnpkg/plugin-allow-scripts.cjs