fix duplicated intersphinx

Author: HumphreyYang

lectures/_config.yml

@@ -44,10 +44,6 @@ sphinx:
         - "https://jax.quantecon.org/"
         - null
     bibtex_reference_style: author_year
-    intersphinx_mapping:
-      intermediate:
-        - "https://python.quantecon.org/"
-        - null
     linkcheck_ignore: ['https://doi.org/10.3982/ECTA8070',
                        'https://doi.org/10.1086/261749',
                        'https://doi.org/10.1086/262078',