fix: reverted fetch from as the field does not exists for some ref doctypes

erpnext/accounts/doctype/payment_entry_reference/payment_entry_reference.json

@@ -88,7 +88,6 @@
    "bold": 0,
    "collapsible": 0,
    "columns": 0,
-   "fetch_from": "reference_name.due_date", 
    "fetch_if_empty": 0,
    "fieldname": "due_date",
    "fieldtype": "Date",
@@ -288,7 +287,6 @@
    "collapsible": 0,
    "columns": 0,
    "depends_on": "eval:(doc.reference_doctype=='Purchase Invoice')",
-   "fetch_from": "reference_name.conversion_rate", 
    "fetch_if_empty": 0,
    "fieldname": "exchange_rate",
    "fieldtype": "Float",