Be sure to use the 'Context menu (right-click) or the small arrow on the delete icon: Remove books of a specific format
otherwise: you could end up deleting ALL of the books that (also) HAVE a AZW3

But why?

If this was the around the year 2000 when disk drives of size were expensive: yes!
This is 2016 where a 2T drive is less than some folks bi-weekly Starbucks tab