VE-573: remove some old code that was breaking category pages on answers wikis

https://wikia-inc.atlassian.net/browse/VE-573

The problem was that for answers wiki's there is class called `CategoryWithAds` which extends `CategoryViewer` and overrides the properties `from` and `until` with strings (they are supposed to be arrays). From what I can tell, the code is old enough that it no longer matches with the underlying `CategoryViewer` class which expects an array of key/value pairs. I spoke with @nefretete about whether this code was necessary anymore and he said it was okay to retire it.