AC_WPNav: use singleton for getting AC_Avoid instance

libraries/AC_WPNav/AC_Loiter.cpp

@@ -305,6 +305,7 @@ void AC_Loiter::calc_desired_velocity(float nav_dt)
 
     // Limit the velocity to prevent fence violations
     // TODO: We need to also limit the _desired_accel
+    AC_Avoid *_avoid = AP::ac_avoid();
     if (_avoid != nullptr) {
         _avoid->adjust_velocity(_pos_control.get_pos_xy_p().kP(), _accel_cmss, desired_vel, nav_dt);
     }

libraries/AC_WPNav/AC_Loiter.h

@@ -16,9 +16,6 @@ public:
     /// Constructor
     AC_Loiter(const AP_InertialNav& inav, const AP_AHRS_View& ahrs, AC_PosControl& pos_control, const AC_AttitudeControl& attitude_control);
 
-    /// provide pointer to avoidance library
-    void set_avoidance(AC_Avoid* avoid_ptr) { _avoid = avoid_ptr; }
-
     /// init_target to a position in cm from ekf origin
     void init_target(const Vector3f& position);
 
@@ -73,7 +70,6 @@ protected:
     const AP_AHRS_View&     _ahrs;
     AC_PosControl&          _pos_control;
     const AC_AttitudeControl& _attitude_control;
-    AC_Avoid                *_avoid = nullptr;
 
     // parameters
     AP_Float    _angle_max;             // maximum pilot commanded angle in degrees. Set to zero for 2/3 Angle Max

libraries/AC_WPNav/AC_WPNav.h

@@ -51,9 +51,6 @@ public:
     /// provide pointer to terrain database
     void set_terrain(AP_Terrain* terrain_ptr) { _terrain = terrain_ptr; }
 
-    /// provide pointer to avoidance library
-    void set_avoidance(AC_Avoid* avoid_ptr) { _avoid = avoid_ptr; }
-
     /// provide rangefinder altitude
     void set_rangefinder_alt(bool use, bool healthy, float alt_cm) { _rangefinder_available = use; _rangefinder_healthy = healthy; _rangefinder_alt_cm = alt_cm; }
 
@@ -285,7 +282,6 @@ protected:
     AC_PosControl&          _pos_control;
     const AC_AttitudeControl& _attitude_control;
     AP_Terrain              *_terrain;
-    AC_Avoid                *_avoid;
 
     // parameters
     AP_Float    _wp_speed_cms;          // default maximum horizontal speed in cm/s during missions