[Stale Content] Delete M-03-13 improper payments guidance — superseded by OMB A-123 Appendix C

## What needs to change
The summary file for OMB M-03-13 (Improper Payments Information Act of 2002, 2003) should be removed. This memo was superseded by OMB Circular A-123 Appendix C (M-18-20), which is already listed in the Guidance category. Keeping both creates confusion about which document agencies should follow.

## Where to find it
Navigate to `pages/_summaries/` and find the file linking to OMB M-03-13. Search if needed:
```
grep -r "M-03-13\|Improper Payments Information Act" pages/_summaries/
```

## Before deleting
Confirm nothing else in the repo links to this file:
```
grep -r "m-03-13" pages/
```

## How to verify
Visit https://resources.data.gov/categories/guidance and confirm the M-03-13 entry no longer appears. Confirm the A-123 Appendix C entry (M-18-20) is still present as the current reference.