You know, that is a good question. I know they are being kept because I can reinstall a recent package without internet connection activity.
I know you can clear dead wood from the file tree with:
sudo apt-get autoclean
And you can clear the cache, where ever it is stored with:
sudo apt-get clean
Insanity: Doing the same thing over and over and each time expecting a different result.
Democracy is 2 wolves and a lamb voting on the menu. Liberty is an armed lamb protesting the electoral outcome. A Republic negates the need for an armed protest.