chore: remove E2E testing and fix CI pnpm build errors

Delete all non-browser E2E testing infrastructure (board directive).
Fix ERR_PNPM_IGNORED_BUILDS by adding pnpm.onlyBuiltDependencies.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/e2e.yaml

@@ -1,23 +0,0 @@
-name: E2E Tests
-
-on:
-  push:
-    branches: [main]
-  pull_request:
-    branches: [main]
-  workflow_dispatch:
-
-permissions:
-  contents: read
-
-concurrency:
-  group: e2e-${{ github.repository }}
-  cancel-in-progress: false
-
-jobs:
-  e2e:
-    uses: privilegedescalation/.github/.github/workflows/plugin-e2e.yaml@hugh/add-pnpm-support-plugin-e2e
-    with:
-      node-version: '22'
-      headlamp-version: v0.40.1
-      e2e-namespace: headlamp-dev