Alternatively, don't bother searching for anything, via the tag browser OR via handcrafted boolean operators (fun though they are).
Just CTRL+A (select all) and right-click Remove books ==> Remove files of a specific format from selected books.
It will popup with a choose dialog where you can select formats (with the number of occurrences listed in brackets) and you can select MBP. Exactly the same as any other process except you only do the step that is actually necessary....
