Drop the missing source/_static from tojupyter_static_file_path (#659)

lectures/source/_static does not exist, so every notebook build logged
"tojupyter_static_path entry .../lectures/source/_static does not exist".
sphinx-tojupyter copies only the entries that exist, so dropping it
changes nothing but the warning. See QuantEcon/actions#300.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WT8bMYFuqCc4A8Y1tGJoXT

Author: mmcky

lectures/_config.yml

@@ -79,7 +79,7 @@ sphinx:
     mathjax_path: https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
     rediraffe_redirects:
       index_toc.md: intro.md
-    tojupyter_static_file_path: ["source/_static", "_static"]
+    tojupyter_static_file_path: ["_static"]
     tojupyter_target_html: true
     tojupyter_urlpath: "https://python-programming.quantecon.org/"
     tojupyter_image_urlpath: "https://python-programming.quantecon.org/_static/"