Drop merge action

Now at https://github.com/flathub-infra/merge-action

.github/workflows/build-merge.yml

@@ -1,46 +0,0 @@
-name: 'Build image for merge workflow'
-
-on:
-  push:
-    branches: master
-    paths:
-      - ".github/actions/merge/*"
-      - .github/workflows/merge.yml
-      - .github/workflows/build-merge.yml
-  workflow_dispatch:
-
-jobs:
-  docker:
-    runs-on: ubuntu-latest
-    permissions:
-      contents: read
-      packages: write
-    steps:
-        # 4.2.2
-      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
-        with:
-          persist-credentials: false
-
-      - name: Set up Docker Buildx
-        # 3.10.0
-        uses: docker/setup-buildx-action@b5ca514318bd6ebac0fb2aedd5d36ec1b5c232a2
-
-      - name: Login to ghcr.io
-        # 3.4.0
-        uses: docker/login-action@74a5d142397b4f367a81961eba4e8cd7edddf772
-        with:
-          registry: ghcr.io
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
-
-      - name: Build and push
-        id: docker_build
-        # 6.15.0
-        uses: docker/build-push-action@471d1dc4e07e5cdedd4c2171150001c434f0b7a4
-        with:
-          context: .github/actions/merge
-          file: .github/actions/merge/Dockerfile
-          tags: ghcr.io/flathub/actions/merge:latest
-          push: true
-          cache-from: type=gha
-          cache-to: type=gha,mode=max