Restore legacy behaviour at game end for multi-player skirmish games

Since it seems that the end-game dialog is generally not appreciated in the multiplayer community ([[1](https://github.com/OpenRA/OpenRA/issues/6312)], [[2](https://github.com/OpenRA/OpenRA/issues/6422)], [[3](http://logs.openra.net/?year=2014&month=08&day=25#02:31:05)), I think the best way forward is to disable it outright for multiplayer games.

It will still show for missions and single-player skirmishes, because I say so.

Reverts the bulk of #6397 as well, since the feature is out of place where it appears now.

Fixes #6422.