delete_port mistakenly uses string as input to _delete_ports_flowrules_by_id

[norman shen]

https://github.com/openstack/networking-sfc/blob/b361517b04adbc7038b385a91e5f9d0e4ffe3a2a/networking_sfc/services/sfc/agent/extensions/sfc.py#L116

```python
    def delete_port(self, context, port):
        """Handle agent SFC extension port delete."""
        port_id = port['port_id']
        resync = False
        LOG.info("a device %s is removed", port_id)
        try:
            self._delete_ports_flowrules_by_id(context, port_id)
        except Exception as e:
            LOG.exception(e)
            LOG.error(
                "delete port flow rule failed for %(port_id)s",
                {'port_id': port_id}
            )
            resync = True

        return
```

function delete_port mistakenly call _delete_ports_flowrules_by_id with port_id as input, where it should be a list. As a result, delete_ports_flowrules_by_id will iterates through each character of the uuid which is weird.