AP_RPM: Use SITL singleton

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

libraries/AP_RPM/RPM_SITL.cpp

@@ -24,9 +24,9 @@ extern const AP_HAL::HAL& hal;
    open the sensor in constructor
 */
 AP_RPM_SITL::AP_RPM_SITL(AP_RPM &_ap_rpm, uint8_t _instance, AP_RPM::RPM_State &_state) :
-	AP_RPM_Backend(_ap_rpm, _instance, _state)
+    AP_RPM_Backend(_ap_rpm, _instance, _state),
+    sitl(AP::sitl())
 {
-    sitl = (SITL::SITL *)AP_Param::find_object("SIM_");
     instance = _instance;
 }