Background: I wanted to see `categories.latest_by` translation in context in a live app but couldn't find it, so I traced it throughout the code.
My step-by-step reasoning for the removal is:
1. `categories-only` does not use `latestTopicOnly`, so there's no need to call it with that argument
2. `parent-category-row` is never called with `latestTopicOnly` argument, so the reference to that arg can be removed from its template
3. after that, `featured-topic` is now no longer ever called with `latestTopicOnly` argument (except in the `ghost` theme, but that's because its override of `categories-only` template 4e2fba963c/common/header.html (L119) is based on the old version of that template from core), so it seems safe to remove it there too (`categories.latest_by` i18n string is also no longer needed)
4. then, nothing is using `latestTopicOnly` anymore so it can be removed from `categories` hbs/js
I checked in each step that there are no plugins or themes (in all-the-plugins/all-the-themes) using those properties/arguments/strings.
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