net-sysfs: remove unused fmt_long_hex

Ever since commit 04ed3e74
("net: change netdev->features to u32") the format string
fmt_long_hex has not been used, so we may as well remove it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>