Plane: remove setting accel_z PID controller's dt

This is handled within pos-controller's set_dt function

ArduPlane/quadplane.cpp

@@ -534,7 +534,6 @@ bool QuadPlane::setup(void)
     motors->set_throttle_range(thr_min_pwm, thr_max_pwm);
     motors->set_update_rate(rc_speed);
     motors->set_interlock(true);
-    pos_control->get_accel_z_pid().set_dt(loop_delta_t);
     pos_control->set_dt(loop_delta_t);
     attitude_control->parameter_sanity_check();