nic_bridge: drop 'gratuitous ARP' messages

'Gratuitous ARP' broadcast messages are used to announce newly created
IP<->MAC address mappings to other hosts. nic_bridge-internal hosts
would expect a nic_bridge-internal MAC address in this message, whereas
external hosts would expect the NIC's MAC address in this message.
The simplest solution to this problem is to just drop those messages,
since they are not really necessary.

Fixes #709.