AP_NavEKF2: review fixes

thanks Paul!

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -684,6 +684,9 @@ bool NavEKF2::InitialiseFilter(void)
         primary = 0;
     }
 
+    // invalidate shared origin
+    common_origin_valid = false;
+    
     // initialise the cores. We return success only if all cores
     // initialise successfully
     bool ret = true;

libraries/AP_NavEKF2/AP_NavEKF2_Control.cpp

@@ -432,7 +432,6 @@ bool NavEKF2_core::setOriginLLH(const Location &loc)
 // Set the NED origin to be used until the next filter reset
 void NavEKF2_core::setOrigin(const Location &loc)
 {
-    // assume origin at current GPS location (no averaging)
     EKF_origin = loc;
     // if flying, correct for height change from takeoff so that the origin is at field elevation
     if (inFlight) {