Remove rkt support (#4671)

README.md

@@ -111,7 +111,6 @@ Supported Components
     -   [kubernetes](https://github.com/kubernetes/kubernetes) v1.14.1
     -   [etcd](https://github.com/coreos/etcd) v3.2.26
     -   [docker](https://www.docker.com/) v18.06 (see note)
-    -   [rkt](https://github.com/rkt/rkt) v1.21.0 (see Note 2)
     -   [cri-o](http://cri-o.io/) v1.11.5 (experimental: see [CRI-O Note](docs/cri-o.md). Only on centos based OS)
 -   Network Plugin
     -   [calico](https://github.com/projectcalico/calico) v3.4.0
@@ -131,11 +130,6 @@ Supported Components
 
 Note: The list of validated [docker versions](https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.13.md) was updated to 1.11.1, 1.12.1, 1.13.1, 17.03, 17.06, 17.09, 18.06. kubeadm now properly recognizes Docker 18.09.0 and newer, but still treats 18.06 as the default supported version. The kubelet might break on docker's non-standard version numbering (it no longer uses semantic versioning). To ensure auto-updates don't break your cluster look into e.g. yum versionlock plugin or apt pin).
 
-Note 2: rkt support as docker alternative is limited to control plane (etcd and
-kubelet). Docker is still used for Kubernetes cluster workloads and network
-plugins' related OS services. Also note, only one of the supported network
-plugins can be deployed for a given single cluster.
-
 Requirements
 ------------