docs: def prune(self, vault_client):
          """Will remove any mount point which is not actually defined
          in this context. """
          existing = getattr(vault_client,
                             SecretBackend.list_fun)()['data'].items()
          for mount_name, _values in existing:
              # ignore system paths and cubbyhole
              mount_path = normalize_vault_path(mount_name)
              if mount_path.startswith('sys') or mount_path == 'cubbyhole':
                  continue
  
              exists = [resource.path
                        for resource in self.mounts()
                        if normalize_vault_path(resource.path) == mount_path]
  
              if not exists:
                  LOG.info("removed unknown mount %s", mount_path)
                  getattr(vault_client, SecretBackend.unmount_fun)(mount_path)