fix: Remove redundant variable declaration (#22641)

diff --git a/erpnext/utilities/transaction_base.py b/erpnext/utilities/transaction_base.py
index 5bf85d1..c8e3330 100644
--- a/erpnext/utilities/transaction_base.py
+++ b/erpnext/utilities/transaction_base.py
@@ -121,7 +121,6 @@
 
 	def validate_rate_with_reference_doc(self, ref_details):
 		buying_doctypes = ["Purchase Order", "Purchase Invoice", "Purchase Receipt"]
-		selling_doctypes = ["Sales Invoice", "Delivery Note"]
 
 		if self.doctype in buying_doctypes:
 			to_disable = "Maintain same rate throughout Purchase cycle"