Sub: replace location_offset() and get_distance() function calls with Location object member function calls

This allows removing duplicated code

ArduSub/commands.cpp

@@ -99,5 +99,5 @@ bool Sub::far_from_EKF_origin(const Location& loc)
 {
     // check distance to EKF origin
     const struct Location &ekf_origin = inertial_nav.get_origin();
-    return (get_distance(ekf_origin, loc) > EKF_ORIGIN_MAX_DIST_M);
+    return (ekf_origin.get_distance(loc) > EKF_ORIGIN_MAX_DIST_M);
 }