fix: drop unsupported nfs-retain StorageClass via extraObjects

nfs-subdir-external-provisioner chart v4.0.18 has no extraObjects
values key - it was silently ignored, so nfs-retain never actually
existed. Only nfs-delete (native storageClass.* key) is real.
Updated README expected output to match.

manifests/nfs-provisioner/values.yaml

@@ -9,15 +9,3 @@ storageClass:
   defaultClass: true
   reclaimPolicy: Delete
   archiveOnDelete: false
-
-# Second StorageClass with Retain policy is created via extraObjects below
-extraObjects:
-  - apiVersion: storage.k8s.io/v1
-    kind: StorageClass
-    metadata:
-      name: nfs-retain
-    provisioner: cluster.local/nfs-subdir-external-provisioner
-    parameters:
-      archiveOnDelete: "false"
-    reclaimPolicy: Retain
-    volumeBindingMode: Immediate