don't enter src MACs of forwarded packets in our cache

There is no point, and it populates the cache with entries we don't
need. Local containers will still need to issue an ARP request when
they first talk to an IP corresponding to the MAC, so we learn about
the MAC at that point, just as normally happens.

router/router.go

@@ -247,9 +247,6 @@ func (router *Router) handleUDPPacketFunc(dec *EthernetDecoder, po PacketSink) F
 			if decodedLen == 0 {
 				return nil
 			}
-			if router.Macs.Enter(srcMac, srcPeer) {
-				log.Println("Discovered remote MAC", srcMac, "at", srcPeer.Name)
-			}
 			if df {
 				router.LogFrame("Relaying DF", frame, &dec.eth)
 			} else {