Delete committed dataset orphans (Track X) (#627)

These files are committed but read by nothing: the audit at https://quantecon.github.io/data-lectures/audit.json (2026-08-31 run) lists them under orphans, and an authenticated content sweep of 407 repositories (283 QuantEcon org repos plus the forks of the six holding repos, by basename, positive and negative controls in the same pass) on 2026-09-01 found no reader of these paths. Tracker: QuantEcon/workspace-lectures#57.

python_advanced_features/test_table.csv is deliberately kept: the exercise prose at python_advanced_features.md tells readers to download that exact URL. The python_foundations copy is a byte-identical duplicate that nothing links to; numbers.txt and us_cities.txt are written by the lectures themselves with %%file / %%writefile; ticker_data.csv is referenced by nothing.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky