CI: Remove unused docker buildx #10293

Job	Run time
build_minikube_test_binaries	7m 48s
build_minikube	14m 19s
lint	5m 39s
unit_test	4m 9s
functional_docker_containerd_ubuntu	9m 42s
functional_docker_ubuntu	11m 40s
functional_podman_ubuntu	11m 59s
functional_docker_rootless_containerd_ubuntu	14m 50s
functional_baremetal_ubuntu20_04	7m 18s
functional_virtualbox_macos	6s
upload_all_reports	18s
	1h 27m 48s

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