kubelet: remove the --docker-exec-handler flag

Stop supporting the "nsenter" exec handler. Only the Docker native exec
handler is supported.

The flag was deprecated in Kubernetes 1.6 and is safe to remove
in Kubernetes 1.9 according to the deprecation policy.

pkg/kubelet/dockershim/docker_service.go

@@ -147,22 +147,12 @@ var internalLabelKeys []string = []string{containerTypeLabelKey, containerLogPat
 
 // NOTE: Anything passed to DockerService should be eventually handled in another way when we switch to running the shim as a different process.
 func NewDockerService(client libdocker.Interface, podSandboxImage string, streamingConfig *streaming.Config,
-	pluginSettings *NetworkPluginSettings, cgroupsName string, kubeCgroupDriver string, execHandlerName, dockershimRootDir string, disableSharedPID bool) (DockerService, error) {
+	pluginSettings *NetworkPluginSettings, cgroupsName string, kubeCgroupDriver string, dockershimRootDir string, disableSharedPID bool) (DockerService, error) {
 	c := libdocker.NewInstrumentedInterface(client)
 	checkpointHandler, err := NewPersistentCheckpointHandler(dockershimRootDir)
 	if err != nil {
 		return nil, err
 	}
-	var execHandler ExecHandler
-	switch execHandlerName {
-	case "native":
-		execHandler = &NativeExecHandler{}
-	case "nsenter":
-		execHandler = &NsenterExecHandler{}
-	default:
-		glog.Warningf("Unknown Docker exec handler %q; defaulting to native", execHandlerName)
-		execHandler = &NativeExecHandler{}
-	}
 
 	ds := &dockerService{
 		client:          c,
@@ -170,7 +160,7 @@ func NewDockerService(client libdocker.Interface, podSandboxImage string, stream
 		podSandboxImage: podSandboxImage,
 		streamingRuntime: &streamingRuntime{
 			client:      client,
-			execHandler: execHandler,
+			execHandler: &NativeExecHandler{},
 		},
 		containerManager:  cm.NewContainerManager(cgroupsName, client),
 		checkpointHandler: checkpointHandler,