Remove unused src/native/managed/cdacreader directory (#124675)

The `src/native/managed/cdacreader` directory was an empty/stale
artifact containing only a `Directory.Build.targets` that forwarded to
its parent — no projects, no source, no references from any build
scripts.

## Changes

- Deleted `src/native/managed/cdacreader/Directory.Build.targets` (and
the now-empty directory)

Author: Copilot