[Stale Content] Delete National Broadband Map case study — program no longer exists

## What needs to change
The National Broadband Map case study links to a Scribd document describing a broadband mapping program that no longer exists in its original form. The FCC launched an entirely new broadband map in 2022 using a different methodology and platform. The case study describes a program that is no longer operative and should be removed.

## Where to find it
Navigate to `pages/_summaries/` and find the file linking to the Scribd document about the National Broadband Map. Search if needed:
```
grep -r "National Broadband Map\|scribd.com.*broadband" pages/_summaries/
```

## Before deleting
Confirm nothing else in the repo links to this file:
```
grep -r "national-broadband-map\|broadband-map" pages/
```

## How to verify
Visit https://resources.data.gov/categories/case-studies-examples and confirm the National Broadband Map case study no longer appears in the listing.