Fixes vanilla bug/inconsistency in harvester replacement logic

[ShizCalev]

Vanilla behavior is SUPPOSED to be that Easy AI are never supposed to replace their harvesters (it's explicitly stated in TD's code for the campaign AI here): https://github.com/ChthonVII/CnC_Remastered_Collection/blob/a432e30081ea6228b3e4eebf398a9d82fa545d81/TIBERIANDAWN/HOUSE.CPP#L3805-L3810

However, it appears this was accidentally inverted in RA's equivalent build logic - checking if the AI's difficulty is not set to hard, meaning that Hard AI will not replace their harvesters instead. This mistake then got backported to TD for the skirmish AI with the remaster.

This is an obvious inconsistency/mistake resulting in the Hard AI actually being easier to defeat by just targeting their harvesters, and the easy AI being harder than intended.