refactor: removed unrequired code and test for standalone delivery note serial return (#30276)

diff --git a/erpnext/stock/doctype/delivery_note/delivery_note.py b/erpnext/stock/doctype/delivery_note/delivery_note.py
index 7951b8d..ffa2f93 100644
--- a/erpnext/stock/doctype/delivery_note/delivery_note.py
+++ b/erpnext/stock/doctype/delivery_note/delivery_note.py
@@ -13,10 +13,7 @@
 from erpnext.controllers.accounts_controller import get_taxes_and_charges
 from erpnext.controllers.selling_controller import SellingController
 from erpnext.stock.doctype.batch.batch import set_batch_nos
-from erpnext.stock.doctype.serial_no.serial_no import (
-	get_delivery_note_serial_no,
-	update_serial_nos_after_submit,
-)
+from erpnext.stock.doctype.serial_no.serial_no import get_delivery_note_serial_no
 
 form_grid_templates = {
 	"items": "templates/form_grid/item_grid.html"
@@ -218,9 +215,6 @@
 		# Updating stock ledger should always be called after updating prevdoc status,
 		# because updating reserved qty in bin depends upon updated delivered qty in SO
 		self.update_stock_ledger()
-		if self.is_return:
-			update_serial_nos_after_submit(self, "items")
-
 		self.make_gl_entries()
 		self.repost_future_sle_and_gle()
 
diff --git a/erpnext/stock/doctype/delivery_note/test_delivery_note.py b/erpnext/stock/doctype/delivery_note/test_delivery_note.py
index 9dc112f..82f4e7d 100644
--- a/erpnext/stock/doctype/delivery_note/test_delivery_note.py
+++ b/erpnext/stock/doctype/delivery_note/test_delivery_note.py
@@ -823,11 +823,6 @@
 
 		automatically_fetch_payment_terms(enable=0)
 
-	def test_standalone_serial_no_return(self):
-		dn = create_delivery_note(item_code="_Test Serialized Item With Series", is_return=True, qty=-1)
-		dn.reload()
-		self.assertTrue(dn.items[0].serial_no)
-
 def create_delivery_note(**args):
 	dn = frappe.new_doc("Delivery Note")
 	args = frappe._dict(args)