Remove scaffolder gh workflow which publishes to gh docker registry

We are currently publishing the backstage-cookiecutter to DockerHub.
This is done by an integration between DockerHub and GitHub.
And so, we do not need this workflow and publish to GitHub's registry.

.github/workflows/scaffolder.yml

@@ -1,33 +0,0 @@
-name: Scaffolder
-
-on:
-  pull_request:
-    paths:
-      - '.github/workflows/scaffolder.yml'
-      - './plugins/scaffolder-backend/scripts'
-  push:
-    branches: [master]
-jobs:
-  build:
-    runs-on: ${{ matrix.os }}
-
-    strategy:
-      matrix:
-        os: [ubuntu-latest]
-        python-version: [3.7]
-
-    name: Build Container
-    steps:
-      - uses: actions/checkout@v2
-      # Build Docker Image
-      - name: Build and push Docker images
-        uses: docker/build-push-action@v1.1.0
-        with:
-          path: plugins/scaffolder-backend/scripts
-          dockerfile: plugins/scaffolder-backend/scripts/Cookiecutter.dockerfile
-          registry: docker.pkg.github.com
-          repository: ${{ github.repository }}/cookiecutter
-          username: ${{ github.actor }}
-          password: ${{ github.token }}
-          tag_with_ref: true
-          push: true