github: disable broken attempt to auto-merge Dependabot PRs again

Sigh, I thought it was fixed now, but it seems it's only `gh pr ready`
that works with `GITHUB_TOKEN`. This rolls back commit ee7e7e1b62.

.github/workflows/dependabot.yml

@@ -1,20 +0,0 @@
-name: build
-
-on:
-  pull_request:
-
-permissions: read-all
-
-jobs:
-  dependabot-auto-merge:
-    name: 'Dependabot auto-merge'
-    permissions:
-      pull-requests: write
-    runs-on: ubuntu-latest
-    if: ${{ github.actor == 'dependabot[bot]' }}
-    steps:
-    - name: Enable auto-merge for Dependabot PRs
-      run: gh pr merge --auto --rebase "$PR_URL"
-      env:
-        PR_URL: ${{github.event.pull_request.html_url}}
-        GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}