remove route headers from external initial requests

(cherry picked from commit f567093efe)

kamailio/default.cfg

@@ -509,7 +509,9 @@ route[PREPARE_INITIAL_REQUESTS]
         break;
     }
 
-    loose_route();
+    if (!isflagset(FLAG_INTERNALLY_SOURCED)) {
+        remove_hf("Route");
+    }
 
     if(!isflagset(FLAG_INTERNALLY_SOURCED)) {
         record_route();