Plane: allow battery failsafe with usb connected

some people connect USB to allow connections from a companion
computer. The arming check is sufficient to prevent unwanted battery
failsafes when bench testing

ArduPlane/ArduPlane.cpp

@@ -52,7 +52,6 @@ const AP_Scheduler::Task Plane::scheduler_tasks[] = {
     SCHED_TASK(gcs_update,             50,    500),
     SCHED_TASK(gcs_data_stream_send,   50,    500),
     SCHED_TASK(update_events,          50,    150),
-    SCHED_TASK(check_usb_mux,          10,    100),
     SCHED_TASK(read_battery,           10,    300),
     SCHED_TASK(compass_accumulate,     50,    200),
     SCHED_TASK(barometer_accumulate,   50,    150),

ArduPlane/GCS_Mavlink.cpp

@@ -2163,7 +2163,6 @@ void Plane::mavlink_delay_cb()
         last_5s = tnow;
         gcs_send_text(MAV_SEVERITY_INFO, "Initialising APM");
     }
-    check_usb_mux();
 
     in_mavlink_delay = false;
 }

ArduPlane/Plane.h

@@ -284,9 +284,6 @@ private:
     // RSSI 
     AP_RSSI rssi;      
 
-    // remember if USB is connected, so we can adjust baud rate
-    bool usb_connected;
-
     // This is the state of the flight control system
     // There are multiple states defined such as MANUAL, FBW-A, AUTO
     enum FlightMode control_mode = INITIALISING;
@@ -982,7 +979,6 @@ private:
     void startup_INS_ground(void);
     void update_notify();
     void resetPerfData(void);
-    void check_usb_mux(void);
     void print_comma(void);
     bool should_log(uint32_t mask);
     int8_t throttle_percentage(void);

ArduPlane/sensors.cpp

@@ -118,8 +118,7 @@ void Plane::read_battery(void)
     battery.read();
     compass.set_current(battery.current_amps());
 
-    if (!usb_connected && 
-        hal.util->get_soft_armed() &&
+    if (hal.util->get_soft_armed() &&
         battery.exhausted(g.fs_batt_voltage, g.fs_batt_mah)) {
         low_battery_event();
     }

ArduPlane/system.cpp

@@ -165,11 +165,6 @@ void Plane::init_ardupilot()
 
     rpm_sensor.init();
 
-    // we start by assuming USB connected, as we initialed the serial
-    // port with SERIAL0_BAUD. check_usb_mux() fixes this if need be.    
-    usb_connected = true;
-    check_usb_mux();
-
     // setup telem slots with serial ports
     gcs().setup_uarts(serial_manager);
 
@@ -680,18 +675,6 @@ void Plane::resetPerfData(void)
 }
 
 
-void Plane::check_usb_mux(void)
-{
-    bool usb_check = hal.gpio->usb_connected();
-    if (usb_check == usb_connected) {
-        return;
-    }
-
-    // the user has switched to/from the telemetry port
-    usb_connected = usb_check;
-}
-
-
 void Plane::print_flight_mode(AP_HAL::BetterStream *port, uint8_t mode)
 {
     switch (mode) {