GCS_MAVLink: use visual odometry singleton

libraries/GCS_MAVLink/GCS.h

@@ -291,7 +291,6 @@ protected:
     // enforcement of GCS sysid
     bool accept_packet(const mavlink_status_t &status, mavlink_message_t &msg);
     virtual AP_AdvancedFailsafe *get_advanced_failsafe() const { return nullptr; };
-    virtual AP_VisualOdom *get_visual_odom() const { return nullptr; }
     virtual bool set_mode(uint8_t mode) = 0;
     void set_ekf_origin(const Location& loc);
 

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -24,6 +24,7 @@
 #include <AP_BLHeli/AP_BLHeli.h>
 #include <AP_Common/Semaphore.h>
 #include <AP_Scheduler/AP_Scheduler.h>
+#include <AP_VisualOdom/AP_VisualOdom.h>
 
 #include "GCS.h"
 
@@ -2772,7 +2773,7 @@ void GCS_MAVLINK::handle_data_packet(mavlink_message_t *msg)
 
 void GCS_MAVLINK::handle_vision_position_delta(mavlink_message_t *msg)
 {
-    AP_VisualOdom *visual_odom = get_visual_odom();
+    AP_VisualOdom *visual_odom = AP::visualodom();
     if (visual_odom == nullptr) {
         return;
     }