Remove exercise_style config to test default behavior

- Testing default sphinx-exercise behavior without custom styling
- Will compare results with previous exercise_style configuration

Author: mmcky

lectures/_config.yml

@@ -122,8 +122,6 @@ sphinx:
       complex_and_trig: https://intro.quantecon.org/complex_and_trig.html
     # sphinx-proof
     proof_minimal_theme: true
-    # sphinx-exercise
-    exercise_style: "solution_follow_exercise"
     # sphinx-tojupyter
     tojupyter_static_file_path: ["source/_static", "_static"]
     tojupyter_target_html: true