fix(selling): cancel sales order with cancelled PE

Allow cancelling sales order with cancelled payment entry. Ignoring GL
entries while cancelling the document is required to cancel it, reverse
entries are created by accounts controller.

Related issue: ISS-20-21-09586

erpnext/selling/doctype/sales_order/sales_order.py

@@ -180,6 +180,7 @@ class SalesOrder(SellingController):
 			update_coupon_code_count(self.coupon_code,'used')
 
 	def on_cancel(self):
+		self.ignore_linked_doctypes = ('GL Entry', 'Stock Ledger Entry')
 		super(SalesOrder, self).on_cancel()
 
 		# Cannot cancel closed SO