Fix Atlas warning on reinit.

AIDiff is undefined when calling InitGame. But this cheat stuff should not be in the helper.
(Now people don't need to change the helper if they want a non-cheating AI. @Silier)

This patch is favoured over @trompetin17's by Wilfy: https://irclogs.wildfiregames.com/%230ad-dev/2023-01-16-QuakeNet-%230ad-dev.log @ 21:14.

Reported by: @Langbart
Differential revision: https://code.wildfiregames.com/D4881
Fixes #6302