Clean Skirmish maps of default and invalid gamesetting values.

• The gamesetup class rewrite in D2483 will enable Skirmish and Random maps to fix AIs and Civs by specifying them. Hence remove them from all maps that don't intend to fix them (i.e. all except Egypt 3v3).
• Remove default values (and the few irrelevant non-default values) so that the player-chosen settings may be persisted between Skirmish map selection changes, refs #3120, D2483.
• Remove invalid settings: mapType, AISeed and random map settings Script, Size, Seed, Nomad, BaseHeight, BaseTerrain, refs rP20726, rP21274.
• Remove StartingCamera values since they are useless, refs #4839, D1098.
• Add the "new" keyword to Atlas Valleys and Vesuvius from rP23278, so they appear in the "new" filter.
• Unhide Barcania (3)