Delete the three migrated wave-C2 copies (Track C) (#376)

All three now serve from data-lectures: the repoint merged in #375 and
published in publish-2026aug18, whose notebooks and the .notebooks
mirror both carry the new URLs (verified by basename sweep — one
data-lectures read per notebook, zero refs/heads or loadmat residue).
fred_data.csv was the live runtime-fetch case, which is why deletion
waited for the publish.

Deletion-time re-derivation, 2026-08-18: a fresh authenticated Trees
sweep over all 278 org repos (278 covered, 0 skipped) finds no other
reader of these paths — every other holder of these basenames owns its
own copy (lecture-dp / lecture-dp.monorepo / 2026-tom-course for the
ACS file; lecture-tools-techniques / lecture-mapping /
python-lecture-sandpit.myst for the .mat, with tools-techniques
reading its own copy, tracked in QuantEcon/lecture-tools-techniques#11).

match_transport's lecture_specific directory empties with this change;
risk_aversion keeps its three figure PNGs. The published site keeps
serving the two _static copies until the next cache rebuild plus
publish, per the settle policy — the re-audit booked on
QuantEcon/workspace-lectures#40 verifies that; CI previews stop
serving them immediately now that the prune (#374) is in.

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

Author: mmcky