fixup end of scenario saves

<iceiceice_> does anyone know why we have this line?
<iceiceice_> https://github.com/wesnoth/wesnoth/blob/master/src/play_controller.cpp#L752
<iceiceice_> it causes end of scenario saves to be stripped of all units
<iceiceice_> my theory is that it was some kind of hack for some point in time before carryover was implemented properly
<iceiceice_> i propose to remove it in 1.12 and master