Kernel: Stop exposing gateway field

It doesn't make sense after introduction of routing table which allows
having multiple gateways for every interface, and isn't used by any of
the userspace programs now.

Kernel/Net/NetworkTask.cpp

@@ -67,7 +67,6 @@ void NetworkTask_main(void*)
         if (adapter.class_name() == "LoopbackAdapter"sv) {
             adapter.set_ipv4_address({ 127, 0, 0, 1 });
             adapter.set_ipv4_netmask({ 255, 0, 0, 0 });
-            adapter.set_ipv4_gateway({ 0, 0, 0, 0 });
         }
 
         adapter.on_receive = [&]() {