Remove empty/unused CLAUDE.md file. (#1720)

This file was inadvertently added. At this time, we are not using it; if
we need it in the future, we can add it back with some content.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan