unify k8s client in e2e

[nakamasato]

there're two ways in e2e:

1. https://pkg.go.dev/k8s.io/client-go/tools/clientcmd: Package clientcmd provides one stop shopping for building a working client from a fixed config, from a .kubeconfig file, from command line flags, or from any merged combination.

   setup:

   config, err := clientcmd.BuildConfigFromFlags("", kubeconfigPath)
   if err != nil {
   	log.Fatal(err)
   }
   clientset, err := kubernetes.NewForConfig(config)
   if err != nil {
   	log.Fatal(err)
   }

   example usage:

    deployment, err := clientset.AppsV1().Deployments("default").Get(context.TODO(), name, metav1.GetOptions{})

2. https://pkg.go.dev/sigs.k8s.io/controller-runtime/pkg/client: Package client contains functionality for interacting with Kubernetes API servers.
   setup:

   k8sClient, err = client.New(cfg, client.Options{Scheme: scheme.Scheme})
   	  Expect(err).NotTo(HaveOccurred())
   	  Expect(k8sClient).NotTo(BeNil())

   usage:

   Expect(k8sClient.Delete(ctx, mysqlUser)).Should(Succeed())

[nakamasato]

b2e7e53 removed kubectl.go