github: remove broken workflow to enable auto-merge for Dependabot

The workflow that was supposed to enable auto-merge for PRs from
Dependabot is failing like this:


```
Message: Resource not accessible by integration, Locations: [{Line:1 Column:72}]
```

I can't figure out why it's failing (maybe
https://github.com/cli/cli/issues/1314?), so let's just remove it.

.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}}