cleanup: Remove flocker volume plugins from k8s codebase

Flocker storage plugin removed from k8s codebase.

Flocker, an early external storage plugin in k8s,
has not been in maintenance and their business is
down. As far as I know, the plugin is not being
used anymore.

This PR removes the whole flocker dependency and
codebase from core k8s to reduce potential security
risks and reduce maintenance work from the sig-storage community.

cmd/kube-controller-manager/app/plugins.go

@@ -34,7 +34,6 @@ import (
 	"k8s.io/kubernetes/pkg/volume/csi"
 	"k8s.io/kubernetes/pkg/volume/fc"
 	"k8s.io/kubernetes/pkg/volume/flexvolume"
-	"k8s.io/kubernetes/pkg/volume/flocker"
 	"k8s.io/kubernetes/pkg/volume/glusterfs"
 	"k8s.io/kubernetes/pkg/volume/hostpath"
 	"k8s.io/kubernetes/pkg/volume/iscsi"
@@ -133,7 +132,6 @@ func ProbeControllerVolumePlugins(cloud cloudprovider.Interface, config persiste
 		return allPlugins, err
 	}
 
-	allPlugins = append(allPlugins, flocker.ProbeVolumePlugins()...)
 	allPlugins = append(allPlugins, local.ProbeVolumePlugins()...)
 	allPlugins = append(allPlugins, storageos.ProbeVolumePlugins()...)