Remove Obsolete File Metadata Tests for Numeric Prefixes

## Description

The codebase contains test cases for numeric prefix handling in file metadata generation that are no longer needed. These tests are marked with "Partially needed" comments (or other keywords, @AmberFryar?), indicating they were kept for legacy compatibility but be removed once the numeric prefix feature has been deprecated.

**Background:**
The file metadata system previously supported numeric prefixes (e.g., `010_filename.md`, `020_filename.md`) for ordering documentation files. This feature was later removed, but some test cases were retained for backward compatibility or transition purposes.

**Current state:**
- These tests validate behavior for a feature (numeric prefixes) that no longer exists
- Keeping these tests adds maintenance overhead and confusion

**Impact:**
- **Test suite bloat** - Unnecessary tests make the suite harder to understand and maintain
- **Misleading coverage** - Tests for removed features don't provide value

## Related Files

- https://github.com/defenseunicorns/pepr-docs/blob/main/scripts/tests/file-metadata.test.mjs

## Definition of Done

Remove old functionality and transformation from project once older versions of the docs have aged-off.