1. Drop MaxPostsInThread in runtime config, appears to not be used. (also, should'nt clippy warn about this?
2. Same with MaxThreadsInCategory.
3. Tweak this comment which appears to be incorectly describing automated cleanup as being a reason for constraints. Doubl check!

/// Upper bounds for storage maps and double maps. Needed to prevent potential block exhaustion during deletion, etc.
/// MaxSubcategories, and MaxCategories should be reasonably small because when the category is deleted
/// all of it's subcategories with their threads and posts will be iterated over and deleted.

1. Rename MaxCategories to MaxTotalCategories, as people have been confused about meaning => Review limitatinos for forum #3676
2. Rename MaxSubcategories to MaxDirectSubcategoriesInCategory for same reason.

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