Deprecate kubeconfig's preference field in favor of kuberc

Signed-off-by: Maciej Szulik <soltysh@gmail.com>

Kubernetes-commit: 8cf5e8db78deb186ef362f64ab779c09e9520156

pkg/cmd/config/config_test.go

@@ -69,7 +69,6 @@ func Example_view() {
 	//   name: federal-context
 	// current-context: federal-context
 	// kind: Config
-	// preferences: {}
 	// users:
 	// - name: red-user
 	//   user:

pkg/cmd/config/view_test.go

@@ -85,7 +85,6 @@ contexts:
   name: my-cluster
 current-context: minikube
 kind: Config
-preferences: {}
 users:
 - name: minikube
   user:
@@ -165,7 +164,6 @@ contexts:
   name: my-cluster
 current-context: minikube
 kind: Config
-preferences: {}
 users:
 - name: minikube
   user:
@@ -247,7 +245,6 @@ contexts:
   name: minikube
 current-context: minikube
 kind: Config
-preferences: {}
 users:
 - name: minikube
   user:
@@ -272,7 +269,6 @@ contexts:
   name: my-cluster
 current-context: my-cluster
 kind: Config
-preferences: {}
 users:
 - name: mu-cluster
   user: