refactor: remove dead code and stale comments

[henryiii]

🤖 AI text below 🤖

Three small low-risk cleanups tracked in issue #228:

1. Dead code in remote_file() (src/skhep_testdata/remote_files.py): removed the unreachable if not config and raise_missing block — get_config_for_file() already raises RuntimeError for unknown files, so config is never falsy after that call.

2. Stale comments in data_path() (src/skhep_testdata/local_files.py): replaced four speculative/incorrect comment lines (including "Download all not implemented", which is false) with a single accurate comment noting the file is fetched from the main branch on GitHub.

3. Comment-style type annotation in main() (src/skhep_testdata/__main__.py): replaced # type: () -> None comment annotation with the proper def main() -> None: syntax.

All three changes pass uv run --extra test pytest and prek -a --quiet (including mypy strict).

Refs #228

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.43%. Comparing base (321dc7e) to head (21c40d3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   72.51%   73.43%   +0.91%     
==========================================
  Files           4        4              
  Lines         131      128       -3     
==========================================
- Hits           95       94       -1     
+ Misses         36       34       -2     

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[eduardo-rodrigues]

Thanks for the clean-up, Henry.