AC_PosControl_Sub: do not reset accel_z integrator when relaxing

Doing so caused the ROV to dive and slowly go back to the altitude
setpoint

libraries/AC_AttitudeControl/AC_PosControl_Sub.cpp

@@ -100,7 +100,6 @@ void AC_PosControl_Sub::relax_alt_hold_controllers()
     _accel_desired.z = 0.0f;
     _accel_last_z_cms = 0.0f;
     _flags.reset_rate_to_accel_z = true;
-    _pid_accel_z.set_integrator(-_motors.get_throttle_hover() * 1000.0f);
     _accel_target.z = -(_ahrs.get_accel_ef_blended().z + GRAVITY_MSS) * 100.0f;
     _pid_accel_z.reset_filter();
 }