Let the difficulty setting of Jebel Barkal proportionally influence the number of attackers spawned, refs #4963.

Remove iberian starting walls but keep the towers as requested by lobby players.
Update mappreview to include the walls from rP21556.
Use Basic rank, since Gaia doesn't have a TechnologyManager and since the rank bonuses are autoresearched-techs
(noted by temple recently and FeldFeld in a match before the first version of D204).