Remove unnecessary asset validation callback

[Toastbrot236]

Removes the AssetValidationParameters.OnNewAssetRefCallback callback passed to ResourceValidationHelper.ValidateReference(), because after more thinking, I've determined it to be unnecessary. Any function calling ValidateResource() can just simply set its asset reference to ValidatedAssetResult.NewAssetRef after calling the function without much effort. The only reason I initially implemented the callback was so we could iterate asset references using loops, but there won't really be any places where that will be useful. We won't iterate photo images after all if we want to actually implement #977, since the mainline games use TEX for small/medium images and JPEG for large images. Only place where iterating could be useful is when validating face icons, but generally, the idea of iterating the asset references (with additional data like asset type, depending on where the function is used) instead of just duplicating a few lines for simplicity is a little overcomplicated imo, and probably not worth it just for one usecase.