Sub: use singletons in AP_Arming

ArduSub/AP_Arming_Sub.cpp

@@ -28,11 +28,11 @@ bool AP_Arming_Sub::ins_checks(bool display_failure)
         return false;
     }
 
-    // additional plane specific checks
+    // additional sub-specific checks
     if ((checks_to_perform & ARMING_CHECK_ALL) ||
         (checks_to_perform & ARMING_CHECK_INS)) {
-        if (!ahrs.healthy()) {
-            const char *reason = ahrs.prearm_failure_reason();
+        if (!AP::ahrs().healthy()) {
+            const char *reason = AP::ahrs().prearm_failure_reason();
             if (reason == nullptr) {
                 reason = "AHRS not healthy";
             }

ArduSub/AP_Arming_Sub.h

@@ -4,11 +4,8 @@
 
 class AP_Arming_Sub : public AP_Arming {
 public:
-    AP_Arming_Sub(const AP_AHRS &ahrs_ref, Compass &compass,
-                  const AP_BattMonitor &battery)
-        : AP_Arming(ahrs_ref, compass, battery)
-    {
-    }
+
+    AP_Arming_Sub() : AP_Arming() { }
 
     /* Do not allow copies */
     AP_Arming_Sub(const AP_Arming_Sub &other) = delete;

ArduSub/Sub.h

@@ -333,7 +333,7 @@ private:
                            FUNCTOR_BIND_MEMBER(&Sub::handle_battery_failsafe, void, const char*, const int8_t),
                            _failsafe_priorities};
 
-    AP_Arming_Sub arming{ahrs, compass, battery};
+    AP_Arming_Sub arming;
 
     // Altitude
     // The cm/s we are moving up or down based on filtered data - Positive = UP