Fix maps after r26298 / rP26298.

Correctly fetch the civ from the player's Identity.
Also remove the notions of the CivChanged message, since it was effectively deleted already.

Differential revision: https://code.wildfiregames.com/D4486
Reviewed by: @wraitii
Comments by: @bb, @Silier, @smiley, @Stan, @vladislavbelov
Fixes #6444