AP_AdvancedFailsafe: use baro singleton

libraries/AP_AdvancedFailsafe/AP_AdvancedFailsafe.cpp

@@ -325,6 +325,7 @@ AP_AdvancedFailsafe::check_altlimit(void)
     }
 
     // see if the barometer is dead
+    const AP_Baro &baro = AP::baro();
     if (AP_HAL::millis() - baro.get_last_update() > 5000) {
         // the barometer has been unresponsive for 5 seconds. See if we can switch to GPS
         if (_amsl_margin_gps != -1 &&

libraries/AP_AdvancedFailsafe/AP_AdvancedFailsafe.h

@@ -51,9 +51,8 @@ public:
     };
 
     // Constructor
-    AP_AdvancedFailsafe(AP_Mission &_mission, AP_Baro &_baro, const AP_GPS &_gps, const RCMapper &_rcmap) :
+    AP_AdvancedFailsafe(AP_Mission &_mission, const AP_GPS &_gps, const RCMapper &_rcmap) :
         mission(_mission),
-        baro(_baro),
         gps(_gps),
         rcmap(_rcmap),
         _gps_loss_count(0),
@@ -96,7 +95,6 @@ protected:
     enum state _state;
 
     AP_Mission &mission;
-    AP_Baro &baro;
     const AP_GPS &gps;
     const RCMapper &rcmap;