Rover: make SmartRTL mode decide whether to save position or not

This changes things to work like the Copter equivalent

APMrover2/mode.h

@@ -358,7 +358,7 @@ public:
     bool reached_destination() override { return smart_rtl_state == SmartRTL_StopAtHome; }
 
     // save current position for use by the smart_rtl flight mode
-    void save_position(bool save_pos);
+    void save_position();
 
 protected:
 

APMrover2/mode_smart_rtl.cpp

@@ -87,7 +87,8 @@ void ModeSmartRTL::update()
 }
 
 // save current position for use by the smart_rtl flight mode
-void ModeSmartRTL::save_position(bool save_pos)
+void ModeSmartRTL::save_position()
 {
+    const bool save_pos = (rover.control_mode != &rover.mode_smartrtl);
     g2.smart_rtl.update(true, save_pos);
 }

APMrover2/system.cpp

@@ -368,8 +368,7 @@ bool Rover::disarm_motors(void)
 // save current position for use by the smart_rtl mode
 void Rover::smart_rtl_update()
 {
-    const bool save_position = (control_mode != &mode_smartrtl);
+    mode_smartrtl.save_position();
-    mode_smartrtl.save_position(save_position);
 }
 
 // returns true if vehicle is a boat