Restore the ability to `kubectl apply --prune` without -n flag

Before https://github.com/kubernetes/kubernetes/pull/83084, `kubectl
apply --prune` can prune resources in all namespaces specified in
config files.  After that PR got merged, only a single namespace is
considered for pruning.  It is OK if namespace is explicitly specified
by --namespace option, but what the PR does is use the default
namespace (or from kubeconfig) if not overridden by command line flag.
That breaks the existing usage of `kubectl apply --prune` without
--namespace option.  If --namespace is not used, there is no error,
and no one notices this issue unless they actually check that pruning
happens.  This issue also prevents resources in multiple namespaces in
config file from being pruned.

kubectl 1.16 does not have this bug.  Let's see the difference between
kubectl 1.16 and kubectl 1.17.  Suppose the following config file:

```yaml
apiVersion: v1
kind: ConfigMap
metadata:
  creationTimestamp: null
  name: foo
  namespace: a
  labels:
    pl: foo
data:
  foo: bar
---
apiVersion: v1
kind: ConfigMap
metadata:
  creationTimestamp: null
  name: bar
  namespace: a
  labels:
    pl: foo
data:
  foo: bar
```

Apply it with `kubectl apply -f file`.  Then comment out ConfigMap foo
in this file.  kubectl 1.16 prunes ConfigMap foo with the following
command:

$ kubectl-1.16 apply -f file -l pl=foo --prune
configmap/bar configured
configmap/foo pruned

But kubectl 1.17 does not prune ConfigMap foo with the same command:

$ kubectl-1.17 apply -f file -l pl=foo --prune
configmap/bar configured

With this patch, kubectl once again can prune the resource as before.

Kubernetes-commit: 7af3b01f24edfde34e42640ee565a5a6bb66ce26

pkg/cmd/apply/prune.go

@@ -77,9 +77,6 @@ func (p *pruner) pruneAll(o *ApplyOptions) error {
 	}
 
 	for n := range p.visitedNamespaces {
-		if len(o.Namespace) != 0 && n != o.Namespace {
-			continue
-		}
 		for _, m := range namespacedRESTMappings {
 			if err := p.prune(n, m); err != nil {
 				return fmt.Errorf("error pruning namespaced object %v: %v", m.GroupVersionKind, err)