Remove workflow for dependency management

This is now handled by the official Github dependabot

.github/workflows/dependabot.yml

@@ -1,15 +0,0 @@
-name: Dependabot Pub
-
-on: workflow_dispatch
-
-jobs:
-  pub:
-    name: Dependabot Pub
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v1
-      - name: Update
-        uses: IchordeDionysos/dependabot-pub-runner@main
-        with:
-          path: /