server level_ reflects true start of game position

previousy, the [side]'s in level_ were updated with controller changes,
so that observers who join would be up to date, since controller changes
were not stored in history. we now change this, so that controller changes
are stored in history

this is in preparation to move all controller tweaks server side
[replay_start] in server replays should match client replays

we delete [multiplayer] from the level in order to construct the
contents of [replay_start], exactly as is done in mp_game_utils.cpp
here:

https://github.com/wesnoth/wesnoth/blob/a4f47a63c7b2c55fd7d8d3cd425ede256b39acdd/src/mp_game_utils.cpp#L188
fixup
fixup
fixup