ekf2: default to in air and not at rest

- this is a more conservative default if a vehicle isn't set (no land detector running)
 - handled horizontal preflight failures in commander when disarmed
rather than overloading xy_valid and v_xy_valid flags
 - ekf2 no longer depend on arming or standby status

src/modules/ekf2/EKF2.hpp

@@ -267,9 +267,6 @@ private:
 
 	bool _callback_registered{false};
 
-	bool _armed{false};
-	bool _standby{false}; // standby arming state
-
 	hrt_abstime _last_status_flag_update{0};
 	hrt_abstime _last_range_sensor_update{0};