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
Merge "Delete unused 'use_ipv6' config option"