I’ve never used Categories before, and so every post was in the “Unfiled” category. I needed a way to bulk edit categories.
Seems that in WP4.0 you can bulk add posts to categories, but you cannot bulk remove categories from posts.
The Mass Set Categories plugin, and a little but of magic code, did the trick.
In case the magic bit of code (which was found in a user review, of all places) is lost, I’ve captured it here.
Open Mass set Categories in WP Admin, and then run following code in Console: jQuery("input[name$=\\[1\\]").attr("checked", false)
This allows you to tick the box for each post listed on the Mass Set Categories output page. In other words, the perfect way to remove a category for every single post all at once.
If you wanted to remove from only a selection of posts, this is not for you.