GitHub Actions: Remove build-image workflow

We don't need to build an image, so let's remove the failing workflow altogether.

.github/workflows/build-image.yml

@@ -1,43 +0,0 @@
-name: Build container image
-on:
-  workflow_dispatch:
-  push:
-    branches:
-      - 'main'
-    tags:
-      - '*'
-  pull_request:
-    paths:
-      - .github/workflows/build-image.yml
-      - Dockerfile
-jobs:
-  build-image:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - uses: docker/setup-qemu-action@v1
-      - uses: docker/setup-buildx-action@v1
-      - uses: docker/login-action@v1
-        with:
-          registry: ghcr.io
-          username: ${{ github.repository_owner }}
-          password: ${{ secrets.GITHUB_TOKEN }}
-        if: github.event_name != 'pull_request'
-      - uses: docker/metadata-action@v3
-        id: meta
-        with:
-          images: ghcr.io/${{ github.repository_owner }}/catstodon
-          flavor: |
-            latest=true
-          tags: |
-            type=edge,branch=main
-            type=match,pattern=v(.*),group=0
-            type=ref,event=pr
-      - uses: docker/build-push-action@v2
-        with:
-          context: .
-          platforms: linux/amd64,linux/arm64
-          push: ${{ github.event_name != 'pull_request' }}
-          tags: ${{ steps.meta.outputs.tags }}
-          cache-from: type=registry,ref=ghcr.io/${{ github.repository_owner }}/catstodon:latest
-          cache-to: type=inline