[nfs-client]Remove redundant field in the rbac.yaml of nfs-client

[gobomb]

When I run the command kubectl create -f external-storage/nfs-client/deploy/rbac.yaml, I got some error like this:

```
error: error validating "rbac.yaml": error validating data: ValidationError(ClusterRole.rules[4]): missing required field "verbs" in io.k8s.api.rbac.v1.PolicyRule; if you choose to ignore these errors, turn validation off with --validate=false

```

I found that after removing the redundant field - apiGroups: [""], the command works fine.

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[liangxia]

/LGTM

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In response to this:

/LGTM

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[gobomb]

/area nfs-client

/assign @wongma7

[wongma7]

/lgtm

oops, thank you!