Remove unreachable warning on volume name dup

Volume names are validated to be unique and always have been.  The cited
issues are all about apply getting messed up, not the aspiserver
allowing dups.

```
$ k create -f /tmp/bad.yaml
The Deployment "bad-volumes-test" is invalid: spec.template.spec.volumes[1].name: Duplicate value: "config"

$ k apply --server-side -f /tmp/bad.yaml
Error from server: failed to create typed patch object (default/bad-volumes-test; apps/v1, Kind=Deployment): .spec.template.spec.volumes: duplicate entries for key [name="config"]

$ k apply -f /tmp/bad.yaml -o json | jq '.spec.template.spec.volumes'
The Deployment "bad-volumes-test" is invalid: spec.template.spec.volumes[1].name: Duplicate value: "config"
```

pkg/api/pod/warnings_test.go

@@ -254,20 +254,6 @@ func TestWarnings(t *testing.T) {
 				`spec.imagePullSecrets[0].name: invalid empty name ""`,
 			},
 		},
-		{
-			name: "duplicate volume",
-			template: &api.PodTemplateSpec{Spec: api.PodSpec{
-				Volumes: []api.Volume{
-					{Name: "a"},
-					{Name: "a"},
-					{Name: "a"},
-				}},
-			},
-			expected: []string{
-				`spec.volumes[1].name: duplicate name "a"`,
-				`spec.volumes[2].name: duplicate name "a"`,
-			},
-		},
 		{
 			name: "duplicate env",
 			template: &api.PodTemplateSpec{Spec: api.PodSpec{