UPNP: Don't delete mappings when adding mappings

Don't delete previous mappings when adding a new mapping.
This was a old workaround that seems to cause issues.

(cherry picked from commit 2685cc7bb6708df745317fd2f078bc945555e4f8)

modules/upnp/upnp.cpp

@@ -320,8 +320,6 @@ int UPNP::add_port_mapping(int port, int port_internal, String desc, String prot
 		return UPNP_RESULT_NO_GATEWAY;
 	}
 
-	dev->delete_port_mapping(port, proto);
-
 	return dev->add_port_mapping(port, port_internal, desc, proto, duration);
 }