fix(nb): fixed missing reference in crud-web-apps volumes backend

[mathis-marcotte]

I noticed that the "delete_pvc" endpoint was referencing a missing value from the "viewer_utils" import.

When running that code, it would result in this error:

Traceback (most recent call last):
  File "/home/marcoma/Documents/upstreams/kubeflow/components/crud-web-apps/volumes/backend/web-apps-dev/lib/python3.10/site-packages/flask/app.py", line 917, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/marcoma/Documents/upstreams/kubeflow/components/crud-web-apps/volumes/backend/web-apps-dev/lib/python3.10/site-packages/flask/app.py", line 902, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)  # type: ignore[no-any-return]
  File "/home/marcoma/Documents/upstreams/kubeflow/components/crud-web-apps/volumes/backend/apps/default/routes/delete.py", line 30, in delete_pvc
    viewer_utils.VIEWER_LABEL,
AttributeError: module 'apps.common.viewer' has no attribute 'VIEWER_LABEL'

So I replaced it for a value that does exist. Please confirm if this was the original intent for this warning log.

Original PR here: kubeflow/kubeflow#7718

[google-oss-prow]

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