zebra: remove log for RTM_DELNEIGH of unknown mac

cache, don't log the failed removal.
Zebra was generating unhelpful error logs (nothing apparent for
the operator to learn/do based on the info) when an RTM_DELNEIGH
arrived for a MAC that is not in our local MAC cache:
```
var/log/frr/frr.log:2022-07-30T15:05:25.985778-07:00 leaf01 zebra[8956]: Failed to find local MAC cache for bridge bridge vid 219 mac 00:55:01:00:01:b1 at DEL
var/log/frr/frr.log:2022-07-30T15:05:25.985854-07:00 leaf01 zebra[8956]: Failed to find local MAC cache for bridge bridge vid 217 mac 00:55:01:00:01:35 at DEL
var/log/frr/frr.log:2022-07-30T15:05:25.985943-07:00 leaf01 zebra[8956]: Failed to find local MAC cache for bridge bridge vid 203 mac 00:33:01:00:00:ad at DEL
```

This can occur during normal timing events (like reading the kernel fdb
on startup), and during irregular events there isn't anything for an
operator to do.  So let's skip that error log.

Ticket: #3158836

Signed-off-by: Trey Aspelund <[email protected]>

zebra/rt_netlink.c

@@ -3894,10 +3894,6 @@ static int netlink_macfdb_change(struct nlmsghdr *h, int len, ns_id_t ns_id)
 	bmac = zebra_l2_brvlan_mac_find(br_if, vid, &mac);
 	if (bmac)
 		zebra_l2_brvlan_mac_del(br_if, bmac);
-	else
-		zlog_err(
-			"Failed to find local MAC cache for bridge %s vid %u mac %pEA at DEL",
-			br_if->name, vid, &mac);
 
 	return zebra_vxlan_local_mac_del(ifp, br_if, &mac, vid);
 }