ci(seo): drop setup-node from gsc-submit workflow

actions/setup-node@v5 auto-detects pnpm-lock.yaml and tries to enable
pnpm package-manager caching, which fails because pnpm isn't installed
on the runner. The submit script has zero npm dependencies, so the
preinstalled Node on ubuntu-latest is enough.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Mlaz-code

.github/workflows/gsc-submit.yml

@@ -29,10 +29,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
-      - uses: actions/setup-node@v5
-        with:
-          node-version: 22
-
       - name: Submit sitemap
         env:
           GSC_SERVICE_ACCOUNT_KEY: ${{ secrets.GSC_SERVICE_ACCOUNT_KEY }}