Remove extra IPv6 subnet on mgt network

Akanda automatically creates an IPv6 subnet on every network,
but in this case it is using the wrong CIDR.

Change-Id: Ie13a544644a98d795e5287b7e07b2a841326f018
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge pull request #4 from dhellmann/cleanup-ipv6-subnet

Remove extra IPv6 subnet on mgt network