samples/net: Getting rid of common ieee802154 settings altogether.

Only ieee802154 should have been using it, but it ended in various
samples that did not require it anymore once they've been using
net_app. Unlike former samples settings, net_app settings are tied to
net_app, so let's just forget about all of it and silently use net_app.
If something goes wrong in setting net options, it will be a unique
place.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>