[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 response.clone()