https://www.autonomail.com/blog/freeing-disk-space-with-the-packagekit-cache/
PackageKit does not offer a built in way to clean the updates. So we have to resort to a dumb fix. First delete the whole cache directory:
$ rm -r /var/cache/PackageKit
Then for cleanliness, let PackageKit re-download the metadata cache. This just downloads the metadata cache; not the auto updates:
$ sudo pkcon refresh force -c -1