fix [option]s/[events]s in [modification]s and [era]

this was broken by a commite earlier by me.

We now add them to teh scenario in saved_game.cpp and not in the mp
code. The reason is that we stil want them to work if a mp campaign game
was reloaded in sp.
remove mp_game_settings::scenario_data

this is not used anore since we use saved_game::starting_pos now