Remove the Lighthouse GitHub Actions workflow

Keeping Lighthouse CI (apps/site/lighthouserc*.cjs, pnpm run lighthouse)
for manual/local runs, just not on every push/PR.

.github/workflows/lighthouse.yml

@@ -1,25 +0,0 @@
-name: Lighthouse
-
-on:
-  push:
-    branches: [main, astro-migration]
-  pull_request:
-  workflow_dispatch:
-
-jobs:
-  lighthouse:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-
-      - uses: pnpm/action-setup@v4
-
-      - uses: actions/setup-node@v4
-        with:
-          node-version: 22
-          cache: pnpm
-
-      - run: pnpm install --frozen-lockfile --filter site
-
-      - name: Run Lighthouse CI
-        run: pnpm --filter site run lighthouse