net1080: Remove unused function nc_dump_ttl()

The function is not used, removing it fixes the following warning when
building with clang:

drivers/net/usb/net1080.c:271:20: error: unused function
    'nc_dump_ttl' [-Werror,-Wunused-function]

Also remove the definition of TTL_THIS, which is only used in
nc_dump_ttl()

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>