net: openthread: Disable ND/MLD for OpenThread interfaces

OpenThread networks do not use ND or MLD as they have their own set of
protocols to cover this functionality. So far, the use of OpenThread
with Zephyr relied on disabling those protocols support statically with
Kconfig, which prevented OT interfaces from being used along with other
IPv6 interfaces.

With the new network interface flags, it's now possible to disable ND
and MLD on individual interfaces, therefore disable them for OT
specifically.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>