AP_AHRS: Use SITL singleton

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -291,7 +291,7 @@ void AP_AHRS_NavEKF::update_EKF3(void)
 void AP_AHRS_NavEKF::update_SITL(void)
 {
     if (_sitl == nullptr) {
-        _sitl = (SITL::SITL *)AP_Param::find_object("SIM_");
+        _sitl = AP::sitl();
         if (_sitl == nullptr) {
             return;
         }