The password expiration date field is hardcoded to only show in the patron edit screen for superlibrarians. It continues to appear for superlibrarians, even if selected in the BorrowerUnwantedField system preference. We should either make this field observe the BorrowerUnwantedField syspref or remove it from the list of fields in the syspref. Tangentially, that field is currently failing to load the patron's current password expiration date into the edit form, so the date is lost when the patron is saved. That's bug 35796.