refactor: remove unused variable, pe should pull in parent exc rate

1. 'reference_doc' variable is never set. Hence, removing.
2. set_exchange_rate() relies on ref_doc, which was never
set due to point [1]. Replacing it with 'doc'.
3. Sales/Purchase Invoice has 'conversion_rate' field for tracking
exchange rate. Added a get statement for them as well.
diff --git a/erpnext/accounts/doctype/payment_entry/payment_entry.py b/erpnext/accounts/doctype/payment_entry/payment_entry.py
index 44e3e89..89241eb 100644
--- a/erpnext/accounts/doctype/payment_entry/payment_entry.py
+++ b/erpnext/accounts/doctype/payment_entry/payment_entry.py
@@ -400,7 +400,7 @@
 			else:
 				if ref_doc:
 					if self.paid_from_account_currency == ref_doc.currency:
-						self.source_exchange_rate = ref_doc.get("exchange_rate")
+						self.source_exchange_rate = ref_doc.get("exchange_rate") or ref_doc.get("conversion_rate")
 
 			if not self.source_exchange_rate:
 				self.source_exchange_rate = get_exchange_rate(
@@ -413,7 +413,7 @@
 		elif self.paid_to and not self.target_exchange_rate:
 			if ref_doc:
 				if self.paid_to_account_currency == ref_doc.currency:
-					self.target_exchange_rate = ref_doc.get("exchange_rate")
+					self.target_exchange_rate = ref_doc.get("exchange_rate") or ref_doc.get("conversion_rate")
 
 			if not self.target_exchange_rate:
 				self.target_exchange_rate = get_exchange_rate(
@@ -2005,7 +2005,6 @@
 	payment_type=None,
 	reference_date=None,
 ):
-	reference_doc = None
 	doc = frappe.get_doc(dt, dn)
 	over_billing_allowance = frappe.db.get_single_value("Accounts Settings", "over_billing_allowance")
 	if dt in ("Sales Order", "Purchase Order") and flt(doc.per_billed, 2) >= (
@@ -2145,7 +2144,7 @@
 	update_accounting_dimensions(pe, doc)
 
 	if party_account and bank:
-		pe.set_exchange_rate(ref_doc=reference_doc)
+		pe.set_exchange_rate(ref_doc=doc)
 		pe.set_amounts()
 
 		if discount_amount: