validation: clean up the placeholder directories

The atmosphere examples directory was empty and untracked, so it never did
anything. The validation .gitkeep files were redundant once their
directories held real content, and data/tle was described as holding TLE
samples that no script reads. Remove all three and correct the README.

validation/README.md

@@ -34,7 +34,6 @@ validation/
 │   ├── link/            # 链路预算对拍
 │   └── constellation/   # 覆盖几何对拍
 └── data/
-    ├── tle/             # 公开 TLE 样本
     └── expected/        # Rust 侧导出的待比对数据
 ```