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/EKF/ekf.cpp

@@ -82,8 +82,6 @@ void Ekf::reset()
 
 	_prev_dvel_bias_var.zero();
 
-	_control_status.flags.vehicle_at_rest = true;
-
 	resetGpsDriftCheckFilters();
 }