Plane: remove duplicate last mode reason

ArduPlane/Plane.h

@@ -1057,7 +1057,6 @@ private:
     bool set_mode(Mode& new_mode, const ModeReason reason);
     bool set_mode(const uint8_t mode, const ModeReason reason) override;
     bool set_mode_by_number(const Mode::Number new_mode_number, const ModeReason reason);
-    ModeReason _last_reason;
     void check_long_failsafe();
     void check_short_failsafe();
     void startup_INS_ground(void);

ArduPlane/events.cpp

@@ -201,7 +201,7 @@ void Plane::failsafe_short_off_event(ModeReason reason)
     gcs().send_text(MAV_SEVERITY_WARNING, "Short Failsafe Cleared");
     failsafe.state = FAILSAFE_NONE;
     // restore entry mode if desired but check that our current mode is still due to failsafe
-    if ( _last_reason == ModeReason::RADIO_FAILSAFE) { 
+    if (control_mode_reason == ModeReason::RADIO_FAILSAFE) { 
        set_mode_by_number(failsafe.saved_mode_number, ModeReason::RADIO_FAILSAFE_RECOVERY);
        gcs().send_text(MAV_SEVERITY_INFO,"Flight mode %s restored",control_mode->name());
     }

ArduPlane/system.cpp

@@ -204,14 +204,11 @@ void Plane::startup_ground(void)
 
 bool Plane::set_mode(Mode &new_mode, const ModeReason reason)
 {
-    // update last reason
-    const ModeReason last_reason = _last_reason;
-    _last_reason = reason;
 
     if (control_mode == &new_mode) {
         // don't switch modes if we are already in the correct mode.
         // only make happy noise if using a difent method to switch, this stops beeping for repeated change mode requests from GCS
-        if ((reason != last_reason) && (reason != ModeReason::INITIALISED)) {
+        if ((reason != control_mode_reason) && (reason != ModeReason::INITIALISED)) {
             AP_Notify::events.user_mode_change = 1;
         }
         return true;