MAINT: remove pandas_panel migrated to python-programming (#24)

* MAINT: remove pandas_panel migrated to python-programming

* tidy up assets

Author: mmcky

lectures/_static/lecture_specific/pandas_panel/countries.csv

@@ -4,7 +4,6 @@ parts:
 - caption: Elementary Statistics
   numbered: true
   chapters:
-  - file: pandas_panel
   - file: ols
   - file: mle
   - file: prob_matrix