chore: Remove obsolete preview build support

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.

* [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

.circleci/config.yml

@@ -477,7 +477,7 @@ 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

@@ -118,15 +118,6 @@ npmAuditIgnoreAdvisories:
   # Maintenance has stopped and the project will be archived in 2025.
   - 'react-beautiful-dnd (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
     spec: 'https://raw.githubusercontent.com/LavaMoat/LavaMoat/main/packages/yarn-plugin-allow-scripts/bundles/@yarnpkg/plugin-allow-scripts.js'