long_run_growth: read mpd2020.xlsx from data-lectures (#823)

Repoints the Maddison Project Database at the canonical data repository and
removes this repo's own copy, which nothing reads once the URL moves.

The served bytes are byte-identical to the copy deleted here -- both are git
blob d5076da, and the URL returns sha256 f67af0f, matching the manifest at
QuantEcon/data-lectures lectures/mpd2020.xlsx.yml. So this cannot change a
figure; only where the bytes come from changes.

The lecture assigns data_url once and reuses it for both the `Full data` and
`Regional data` reads, so the single line covers all three read sites.

Deleting the local copy is safe only because lecture-wasm's long_run_growth
moves in the same set -- it fetched this exact blob by URL, and would have
404'd the moment this file went. See QuantEcon/data-lectures PLAN.md,
"Repoint rules".

Part of the Track A migration. See QuantEcon/data-lectures#8 and
QuantEcon/data-lectures#4.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/datasets/mpd2020.xlsx

@@ -86,7 +86,7 @@ Our objective in this section is to produce a convenient `DataFrame` instance th
 Here we read the Maddison data into a pandas `DataFrame`:
 
 ```{code-cell} ipython3
-data_url = "https://github.com/QuantEcon/lecture-python-intro/raw/main/lectures/datasets/mpd2020.xlsx"
+data_url = "https://github.com/QuantEcon/data-lectures/raw/main/lectures/mpd2020.xlsx"
 data = pd.read_excel(data_url,
                      sheet_name='Full data')
 data.head()