Revert "github: Pin a specific image for QEMU for Docker"

This reverts commit 3dbfd17270.

This shouldn't be necessary any longer; QEMU in
tonistiigi/binfmt:latest has been updated to 9.2.2, which has
this bug fixed - see
https://github.com/docker/setup-qemu-action/issues/198#issuecomment-2690225370
https://github.com/tonistiigi/binfmt/issues/215#issuecomment-2689340770
and https://gitlab.com/qemu-project/qemu/-/issues/1913.

.github/workflows/docker.yml

@@ -93,8 +93,6 @@ jobs:
       - name: Set up QEMU
         if: ${{matrix.file == 'Dockerfile.toolchain'}}
         uses: docker/setup-qemu-action@v3
-        with:
-          image: tonistiigi/binfmt:qemu-v7.0.0-28
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
       - name: Log in to Docker Hub