kubeadm: remove the deprecated GA CoreDNS feature-gate

The CoreDNS GA feature-gate in kubeadm was deprecated since 1.13.
The k8s policy is to remove the gate 2 releases after it transitions
to GA:
https://kubernetes.io/docs/reference/using-api/deprecation-policy/#deprecation

We kept it around for longer to prevent existing setups from breaking
as it caused minimal maintenance overhead.

cmd/kubeadm/app/cmd/BUILD

@@ -85,7 +85,6 @@ go_test(
         "//cmd/kubeadm/app/apis/output/v1alpha1:go_default_library",
         "//cmd/kubeadm/app/cmd/options:go_default_library",
         "//cmd/kubeadm/app/constants:go_default_library",
-        "//cmd/kubeadm/app/features:go_default_library",
         "//cmd/kubeadm/app/util:go_default_library",
         "//cmd/kubeadm/app/util/config:go_default_library",
         "//cmd/kubeadm/app/util/output:go_default_library",

cmd/kubeadm/app/cmd/init_test.go

@@ -17,7 +17,6 @@ limitations under the License.
 package cmd
 
 import (
-	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -25,7 +24,6 @@ import (
 
 	"k8s.io/apimachinery/pkg/util/sets"
 	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
-	"k8s.io/kubernetes/cmd/kubeadm/app/features"
 )
 
 const (
@@ -86,13 +84,6 @@ func TestNewInitData(t *testing.T) {
 			},
 			expectError: true,
 		},
-		{
-			name: "fail if deprecated feature gates are set",
-			flags: map[string]string{
-				options.FeatureGatesString: fmt.Sprintf("%s=true", features.CoreDNS),
-			},
-			expectError: true,
-		},
 		{
 			name: "fails if invalid preflight checks are provided",
 			flags: map[string]string{

cmd/kubeadm/app/features/features.go

@@ -28,19 +28,12 @@ import (
 )
 
 const (
-
-	// CoreDNS is GA in v1.11
-	CoreDNS = "CoreDNS"
 	// IPv6DualStack is expected to be alpha in v1.16
 	IPv6DualStack = "IPv6DualStack"
 )
 
-var coreDNSMessage = "featureGates:CoreDNS has been removed in v1.13\n" +
-	"\tUse kubeadm-config to select which DNS addon to install."
-
 // InitFeatureGates are the default feature gates for the init command
 var InitFeatureGates = FeatureList{
-	CoreDNS:       {FeatureSpec: featuregate.FeatureSpec{Default: true, PreRelease: featuregate.Deprecated}, HiddenInHelpText: true, DeprecationMessage: coreDNSMessage},
 	IPv6DualStack: {FeatureSpec: featuregate.FeatureSpec{Default: false, PreRelease: featuregate.Alpha}},
 }
 

cmd/kubeadm/test/cmd/init_test.go

@@ -352,10 +352,6 @@ func TestCmdInitFeatureGates(t *testing.T) {
 			name: "no feature gates passed",
 			args: "",
 		},
-		{
-			name: "feature gate CoreDNS=true",
-			args: "--feature-gates=CoreDNS=true",
-		},
 		{
 			name: "feature gate IPv6DualStack=true",
 			args: "--feature-gates=IPv6DualStack=true",