Remove `v2.Problem.*_df`

After #405 , tables are kept separate. Providing the combined dataframe at e.g. petab_problem.condition_df will mean pre-existing code might silently fail because editing petab_problem.condition_df no longer changes the real condition table(s).

Start by searching the repository for the v2.Problem dataframe attributes and read the context from issue #405. Confirm which separate tables replaced the combined dataframes, then remove the combined attributes while preserving the separate-table interface; done means the obsolete *_df properties are no longer provided.