Delete unused 'use_ipv6' config option

[OpenStack Infra]

https://review.openstack.org/130409
commit 1a9d58115be7dfb6b79571d051f7b855229cc6cb
Author: Ruby Loo <email address hidden>
Date: Thu Oct 23 00:55:48 2014 +0000

    Delete unused 'use_ipv6' config option

    The 'use_ipv6' config option is not being used, and Ironic doesn't support
    IPv6 (see https://bugs.launchpad.net/ironic/+bug/1325942). Someone may
    try to set this value to True, and that wouldn't do anything, so it is
    better to delete it and add it back in the future when IPv6 is supported.

    I'm not sure it needs a DocImpact since it never worked, but I'll tag
    it, just in case we want to document that "the 'use_ipv6' config option
    was deleted because it never worked".

    Closes-Bug: #1384509
    DocImpact

    Change-Id: I7704067ca50448ffb0982e653437d151af85471f