Disable Persian Hall & Ishtar gate as buildable structures.
The Kardakes mercenaries, trainable at the Persian Hall, are historically problematic. Few sources exist on Kardakes troops, but they seem to have been rather heavy infantry, and likely not mercenaries. As such, they would be rather redundant with the Immortal.
The elephants are attested, and might be reintroduced later.
Further, the Ishtar Gate is Neo-Babylonian and predates the 0 A.D. timeline. As such, given its limited use, it is removed as a buildable structure.
Patch by: Nescio
Accepted by: borg-, wraitii
Differential Revision: https://code.wildfiregames.com/D2742