fix(perf): index creation on voucher_detail_no (#27866)

voucher_detail_no is supposed to have an index, it was added on
on_doctype_update function of table, however this function is only
called if DocType itself is updated and `on_update` is called on
DocType. Stock ledger Entry doctype hasn't changed since addition of
this index in function.

Before: Lack of this index was causing full table scan in
get_future_sle_to_fix function. (~50 seconds in a reposting job)

After: Single row is fetched (~0.5 second in full reposting job)

Learnings:
1. Add simple indexes via doctype only
2. For complex indexes always change doctype.json file for it to take
   effect.

erpnext/stock/doctype/stock_ledger_entry/stock_ledger_entry.json

@@ -142,6 +142,7 @@
    "oldfieldtype": "Data",
    "print_width": "150px",
    "read_only": 1,
+   "search_index": 1,
    "width": "150px"
   },
   {
@@ -316,7 +317,7 @@
  "in_create": 1,
  "index_web_pages_for_search": 1,
  "links": [],
- "modified": "2020-09-07 11:10:35.318872",
+ "modified": "2021-10-08 12:42:51.857631",
  "modified_by": "Administrator",
  "module": "Stock",
  "name": "Stock Ledger Entry",

erpnext/stock/doctype/stock_ledger_entry/stock_ledger_entry.py

@@ -181,4 +181,3 @@ def on_doctype_update():
 
 	frappe.db.add_index("Stock Ledger Entry", ["voucher_no", "voucher_type"])
 	frappe.db.add_index("Stock Ledger Entry", ["batch_no", "item_code", "warehouse"])
-	frappe.db.add_index("Stock Ledger Entry", ["voucher_detail_no"])