stm32/modnetwork: Don't take netif's down when network is deinited.

It should be up to the NIC itself to decide if the network interface is
removed upon soft reset.  Some NICs can keep the interface up over a soft
reset, which improves usability of the network.

ports/stm32/modnetwork.c

@@ -67,12 +67,6 @@ void mod_network_init(void) {
 }
 
 void mod_network_deinit(void) {
-    #if MICROPY_PY_LWIP
-    for (struct netif *netif = netif_list; netif != NULL; netif = netif->next) {
-        netif_remove(netif);
-    }
-    // TODO there may be some timeouts that are still pending...
-    #endif
 }
 
 void mod_network_register_nic(mp_obj_t nic) {