Drop nb_repository_url, which points at a notebooks repo that does not exist

lectures/_config.yml set nb_repository_url to
https://github.com/QuantEcon/lecture-python.zh-cn.notebooks, which returns 404.
quantecon-book-theme renders its Launch button whenever the key is set,
so every page linked Colab to the missing repository.

Leaves the comment the lecture-python-programming editions use for the
same case; the theme skips notebook-launch links when the key is unset.

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

Author: mmcky

lectures/_config.yml

@@ -82,7 +82,8 @@ sphinx:
       header_organisation_url: https://quantecon.org
       header_organisation: QuantEcon
       repository_url: https://github.com/QuantEcon/lecture-python.myst
-      nb_repository_url: https://github.com/QuantEcon/lecture-python.zh-cn.notebooks
+      # nb_repository_url omitted deliberately: no notebooks repo exists for this
+      # edition yet, and the theme skips notebook-launch links when it is unset
       twitter: quantecon
       twitter_logo_url: https://assets.quantecon.org/img/qe-twitter-logo.png
       og_logo_url: https://assets.quantecon.org/img/qe-og-logo.png