perf: index `return_against` on delivery note (#38827)
There's a multi-column index but that's useful IFF all parts of column
are part of query.
return against on it's own is VERY unique because it's a primary key, we
don't need a multi-column index here.
(cherry picked from commit 8d79365e0ddef42e788f97c668a64c479a803617)
Co-authored-by: Ankush Menat <ankush@frappe.io>