remove spurious WML variable manipulations in playcampaign.cpp

these appear to cause replay corruption, and bugs 19218, 21562

the replay_start cfg is already set correctly as the snapshot
at the start of a game, in playsingle_controller.cpp, in the
play_scenario function:

https://github.com/wesnoth/wesnoth/blob/0aa13d477d156929616a7409010474c4dc6b5439/src/playsingle_controller.cpp#L372
update changelog