nic_router: no checksum update on IP routing

The router used to update IPv4 checksums when routing via an <ip> rule
despite the fact that it doesn't change any IPv4 header fields in this case.

Ref #4555

repos/os/src/server/nic_router/interface.cc

@@ -1326,7 +1326,6 @@ void Interface::_handle_ip(Ethernet_frame          &eth,
 
 			Domain &remote_domain = rule.domain();
 			_adapt_eth(eth, ip.dst(), pkt, remote_domain);
-			ip.update_checksum();
 			remote_domain.interfaces().for_each([&] (Interface &interface) {
 				interface.send(eth, size_guard);
 			});