Remove unused variable

o neutron/services/firewall/drivers/linux/iptables_fwaas.py:FWAAS_CHAIN
  = 'fwaas'

  This variable was added in the following commit.

    commit 7e487160867d2c2059d556d0d3af50bc8e8838fa
    Author: Rajesh Mohan <rajesh.mlists@gmail.com>
    Date:   Fri Jun 21 17:55:08 2013 -0700

        Firewall as a Service (FWaaS) Iptables Driver

  But there is no evidence that was used.

Change-Id: I5633e74e2441b71e3dddaa0c2f6a5804778e21e1
Merge "Remove unused variable"