dhcp: remove assertion in nm_dhcp_client_handle_event()

Technically, g_warn_if_reached() may not be an assertion, according to
glib. However, there is G_DEBUG=fatal-warnings and we want to run with
that.

So this is an assertion to us. Also, logging to stderr/stdout is not a
useful thing to the daemon. Don't do this. Especially, since it depends
on user provided (untrusted) input.

src/core/dhcp/nm-dhcp-client.c

@@ -965,8 +965,7 @@ nm_dhcp_client_handle_event(gpointer      unused,
                     str_options,
                     priv->config.v6.info_only);
             }
-        } else
-            g_warn_if_reached();
+        }
 
         if (l3cd) {
             nm_l3_config_data_set_dhcp_lease_from_options(l3cd,