quantum_adapter: exception when VM deletion

> Traceback (most recent call last):
>   File "/usr/lib/python2.7/dist-packages/gevent/greenlet.py", line 390, in run
>     result = self._run(*self.args, **self.kwargs)
>   File "/opt/stack/ryu/ryu/base/app_manager.py", line 86, in _event_loop
>     handler(ev)
>   File "/opt/stack/ryu/ryu/app/quantum_adapter.py", line 368, in dp_handler
>     ovs_switch.close()
> AttributeError: 'OVSSwitch' object has no attribute 'close'

This is legacy from the old code, so remove calling close method

Reported-by: Yoshihiro Kaneko <y.kaneko__0929@nifty.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>