fix(stock): remove hardcoded letter_head from report

The 'Incorrect Serial and Batch Bundle' report had a hardcoded
letter_head value of 'Test', preventing users from deleting a
Letter Head named 'Test' due to link check.

Standard reports should not reference specific Letter Head names.

Fixes #52569

(cherry picked from commit 99cd29d)

Author: Roxxane

erpnext/stock/report/incorrect_serial_and_batch_bundle/incorrect_serial_and_batch_bundle.json

@@ -9,7 +9,7 @@
  "idx": 0,
  "is_standard": "Yes",
  "json": "{}",
- "letter_head": "Test",
+ "letter_head": null,
  "letterhead": null,
  "modified": "2025-02-03 15:39:47.613040",
  "modified_by": "Administrator",