P2 pilot: read pandas_panel data from data-lectures

Repoint realwage.csv, countries.csv and employ.csv from this repo's own copies
under lectures/_static/lecture_specific/pandas_panel/ to the canonical copies in
QuantEcon/data-lectures, and delete the local copies.

Byte-compare gate (one-time, reproducible from the data-lectures manifests'
sha256): the removed files are byte-identical to the migrated copies, so the
repoint cannot change lecture output.
  realwage.csv  cd60e36dc4a9e7ac1d7859dc06d5666e32f619dc6272be7c60dd0136c77fd511
  countries.csv b334ddac9bf8b1788c6b9abf626959e2ff98a7f40a9dcdaa22e226d84595c3be
  employ.csv    2ff176e99e4a4669b7b24c603fe1889a2061eff4149d4c356fd84175803149d7

Draft until the data lands: the interim URL 404s until QuantEcon/data-lectures#17
merges. Data PR first, lecture PR second — never a non-default ref.

Part of QuantEcon/meta#338. Pairs with QuantEcon/lecture-python.myst (same change).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mmcky