Remove Flatcar Linux Edge os_image option

* Flatcar Linux has not published an Edge channel image since
April 2020 and recently removed mention of the channel from
their documentation https://github.com/kinvolk/Flatcar/pull/345
* Users of Flatcar Linux Edge should move to the stable, beta, or
alpha channel, barring any alternate advice from upstream Flatcar
Linux

aws/flatcar-linux/kubernetes/cl/controller.yaml

@@ -56,7 +56,6 @@ systemd:
         Wants=rpc-statd.service
         [Service]
         Environment=KUBELET_IMAGE=quay.io/poseidon/kubelet:v1.20.4
-        Environment=KUBELET_CGROUP_DRIVER=${cgroup_driver}
         ExecStartPre=/bin/mkdir -p /etc/kubernetes/cni/net.d
         ExecStartPre=/bin/mkdir -p /etc/kubernetes/manifests
         ExecStartPre=/bin/mkdir -p /opt/cni/bin
@@ -85,7 +84,6 @@ systemd:
           --authentication-token-webhook \
           --authorization-mode=Webhook \
           --bootstrap-kubeconfig=/etc/kubernetes/kubeconfig \
-          --cgroup-driver=$${KUBELET_CGROUP_DRIVER} \
           --client-ca-file=/etc/kubernetes/ca.crt \
           --cluster_dns=${cluster_dns_service_ip} \
           --cluster_domain=${cluster_domain_suffix} \