net: openvswitch: Remove unnecessary skb_nfct()

There is no need add 'if (skb_nfct(skb))' assignment, the
nf_conntrack_put() would check it.

Signed-off-by: Yejune Deng <yejunedeng@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>