samples: net: remove wpanusb sample

Clean up before deprecating legacy device support.
The Linux kernel driver for this example has never been upstreamed or
implemented upstream. In general, 15.4 development does not seem to be a
big success, but if it comes up again, the protocol used by the example
will need to be redesigned anyway.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>