AP_NavEKF3: move global state to be on the stack

libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -557,7 +557,7 @@ void NavEKF3_core::readGpsData()
             gpsCheckStatus.bad_fix = false;
 
             // store fix time from previous read
-            secondLastGpsTime_ms = lastTimeGpsReceived_ms;
+            const uint32_t secondLastGpsTime_ms = lastTimeGpsReceived_ms;
 
             // get current fix time
             lastTimeGpsReceived_ms = gps.last_message_time_ms(selected_gps);

libraries/AP_NavEKF3/AP_NavEKF3_core.cpp

@@ -198,7 +198,6 @@ void NavEKF3_core::InitialiseVariables()
     lastTasPassTime_ms = 0;
     lastSynthYawTime_ms = 0;
     lastTimeGpsReceived_ms = 0;
-    secondLastGpsTime_ms = 0;
     lastDecayTime_ms = imuSampleTime_ms;
     timeAtLastAuxEKF_ms = imuSampleTime_ms;
     flowValidMeaTime_ms = imuSampleTime_ms;

libraries/AP_NavEKF3/AP_NavEKF3_core.h

@@ -1001,7 +1001,6 @@ private:
     uint32_t lastTasPassTime_ms;    // time stamp when airspeed measurement last passed innovation consistency check (msec)
     uint32_t lastTimeGpsReceived_ms;// last time we received GPS data
     uint32_t timeAtLastAuxEKF_ms;   // last time the auxiliary filter was run to fuse range or optical flow measurements
-    uint32_t secondLastGpsTime_ms;  // time of second last GPS fix used to determine how long since last update
     uint32_t lastHealthyMagTime_ms; // time the magnetometer was last declared healthy
     bool allMagSensorsFailed;       // true if all magnetometer sensors have timed out on this flight and we are no longer using magnetometer data
     uint32_t lastSynthYawTime_ms;   // time stamp when yaw observation was last fused (msec)