Remove exercise_style config (not supported by sphinx-exercise 1.0.1)

Author: mmcky

lectures/_config.yml

@@ -79,8 +79,6 @@ sphinx:
     suppress_warnings: [mystnb.unknown_mime_type, myst.domains]
     # sphinx-proof
     proof_minimal_theme: true
-    # sphinx-exercise
-    exercise_style: "solution_follow_exercise"
     # -------------
     html_js_files:
       - https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.4/require.min.js