message: core: remove unused function
diff: @@ -53,8 +53,6 @@ public:
  void send_command_with_ack_async(uint16_t command, const MavlinkCommands::Params &params,
  command_result_callback_t callback,
  uint8_t component_id = 0);
  - void retransmit_command_with_ack_async();
  -
  MavlinkCommands::Result set_msg_rate(uint16_t message_id, double rate_hz);
  