AP_NavEKF3: fix potential time-stamping bug

Use a consistent time reference

libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -283,7 +283,7 @@ void NavEKF3_core::readIMUData()
     dtIMUavg = ins.get_loop_delta_t();
 
     // the imu sample time is used as a common time reference throughout the filter
-    imuSampleTime_ms = AP_HAL::millis();
+    imuSampleTime_ms = frontend->imuSampleTime_us / 1000;
 
     // use the nominated imu or primary if not available
     if (ins.use_accel(imu_index)) {

libraries/AP_NavEKF3/AP_NavEKF3_core.cpp

@@ -498,9 +498,6 @@ void NavEKF3_core::UpdateFilter(bool predict)
 
     // TODO - in-flight restart method
 
-    //get starting time for update step
-    imuSampleTime_ms = frontend->imuSampleTime_us / 1000;
-
     // Check arm status and perform required checks and mode changes
     controlFilterModes();