openvswitch: delete the unncessary skb_pull_rcsum call in ovs_ct_nat_execute

The calls to ovs_ct_nat_execute() are as below:

  ovs_ct_execute()
    ovs_ct_lookup()
      __ovs_ct_lookup()
        ovs_ct_nat()
          ovs_ct_nat_execute()
    ovs_ct_commit()
      __ovs_ct_lookup()
        ovs_ct_nat()
          ovs_ct_nat_execute()

and since skb_pull_rcsum() and skb_push_rcsum() are already
called in ovs_ct_execute(), there's no need to do it again
in ovs_ct_nat_execute().

Reviewed-by: Saeed Mahameed <saeed@kernel.org>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>