Fix save to stop overwriting calculated value
Created by: eileenmcnaughton
This removes the code added in https://github.com/civicrm/org.civicrm.module.cividiscount/pull/200 - I can't replicate the original bug without this code now (good) and this is a hack-approach that is breaking https://github.com/civicrm/civicrm-core/pull/19476