Remove VolumeSubpath feature gate

Remove the VolumeSubpath feature gate.

Feature gate convention has been updated since this was introduced to
indicate that they "are intended to be deprecated and removed after a
feature becomes GA or is dropped.".

pkg/kubelet/kubelet_pods_test.go

@@ -56,65 +56,8 @@ import (
 	"k8s.io/kubernetes/pkg/kubelet/cri/streaming/remotecommand"
 	"k8s.io/kubernetes/pkg/kubelet/prober/results"
 	kubetypes "k8s.io/kubernetes/pkg/kubelet/types"
-	"k8s.io/kubernetes/pkg/volume/util/hostutil"
-	"k8s.io/kubernetes/pkg/volume/util/subpath"
 )
 
-func TestDisabledSubpath(t *testing.T) {
-	fhu := hostutil.NewFakeHostUtil(nil)
-	fsp := &subpath.FakeSubpath{}
-	pod := v1.Pod{
-		Spec: v1.PodSpec{
-			HostNetwork: true,
-		},
-	}
-	podVolumes := kubecontainer.VolumeMap{
-		"disk": kubecontainer.VolumeInfo{Mounter: &stubVolume{path: "/mnt/disk"}},
-	}
-
-	cases := map[string]struct {
-		container   v1.Container
-		expectError bool
-	}{
-		"subpath not specified": {
-			v1.Container{
-				VolumeMounts: []v1.VolumeMount{
-					{
-						MountPath: "/mnt/path3",
-						Name:      "disk",
-						ReadOnly:  true,
-					},
-				},
-			},
-			false,
-		},
-		"subpath specified": {
-			v1.Container{
-				VolumeMounts: []v1.VolumeMount{
-					{
-						MountPath: "/mnt/path3",
-						SubPath:   "/must/not/be/absolute",
-						Name:      "disk",
-						ReadOnly:  true,
-					},
-				},
-			},
-			true,
-		},
-	}
-
-	defer featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.VolumeSubpath, false)()
-	for name, test := range cases {
-		_, _, err := makeMounts(&pod, "/pod", &test.container, "fakepodname", "", []string{}, podVolumes, fhu, fsp, nil, false)
-		if err != nil && !test.expectError {
-			t.Errorf("test %v failed: %v", name, err)
-		}
-		if err == nil && test.expectError {
-			t.Errorf("test %v failed: expected error", name)
-		}
-	}
-}
-
 func TestNodeHostsFileContent(t *testing.T) {
 	testCases := []struct {
 		hostsFileName            string