PR 4f47013feb added a validation step to `NetworkCreate` to ensure
no IPv6 subnet could be set on a network if its `EnableIPv6` parameter
is false.
Before that, the daemon was accepting such request but was doing nothing
with the IPv6 subnet.
This validation step is now deleted, and we automatically set
`EnableIPv6` if an IPv6 subnet was specified.
Signed-off-by: Albin Kerouanton <albinker@gmail.com>