Simplify buildx workflow

There doesn't really need to be a 'develop' branch anymore, since all
work is committed directly to 'main', with tags to indicate
production-ready builds.

As a result, the buildx-dev workflow is pretty pointless.

.github/workflows/buildx-dev.yml

@@ -1,28 +0,0 @@
-name: buildx
-
-on:
-  push:
-    branches: develop
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-    steps:
-      - name: checkout code
-        uses: actions/checkout@v2
-      - name: install buildx
-        id: buildx
-        uses: crazy-max/ghaction-docker-buildx@v1
-        with:
-          version: latest
-      - name: log in to docker hub
-        run: |
-          echo "${{ secrets.DOCKER_PASSWORD }}" | \
-            docker login -u "${{ secrets.DOCKER_USERNAME }}" --password-stdin
-      - name: build and push the image
-        run: |
-          docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
-          docker buildx ls
-          docker buildx build --push \
-            --tag benbusby/whoogle-search:buildx-experimental \
-            --platform linux/amd64,linux/arm/v7,linux/arm64 .