clang-tidy: partially fix readability-redundant-declaration

src/modules/mavlink/mavlink_main.cpp

@@ -85,8 +85,6 @@ static Mavlink *_mavlink_instances = nullptr;
  */
 extern "C" __EXPORT int mavlink_main(int argc, char *argv[]);
 
-extern mavlink_system_t mavlink_system;
-
 void mavlink_send_uart_bytes(mavlink_channel_t chan, const uint8_t *ch, int length)
 {
 	Mavlink *m = Mavlink::get_instance(chan);