When migrating the network configuration to NetBox, variables added
to network dictionary for a node needs to be documented there too
or computed by heuristic. As, such, remove unneeded variables is a win.
The ipv6_native variable is:
- always true when any interface has an IPv6 block
- always false when no interface ahs an IPv6 block
So, the ipv6_native doesn't bring any other value excepted to avoid
to loop all the interfaces, but for 1-4 interfaces, there is no real
speed gain there.
Ref T1762