[lowpan] Prevent NAT64 initialization.

This change prevents the NAT64 mechanism from being initialized and
adding an IPv4 address to the LoWPAN interface, since that behavior
was causing some problems with other software on Fuchsia. This code
will be revised before being re-enabled.

Bug: b/293514558
Bug: b/278659448
Change-Id: Ib1951ee81f30ec2e7f0df34ba612649d117de9be
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/895020
Commit-Queue: Robert Quattlebaum <rquattle@google.com>
Reviewed-by: Wez <wez@google.com>
(cherry picked from commit cfdb1ee795bd4470d437b6ed4a9f3830bc7865a5)