Comment 10 for bug 680834

The reason for this is that update manager has switched it's backend to aptdaemon (previously it was synaptic) and aptdaemon can't be configured to delete the downloaded deb files after installing/updating them (synaptic has an option to do this in it's preferences, and that option did work for update-manager too).
Based on this, the problem is that there's currently no option to configure aptdaemon-based tools (software-center, update-manager, etc) to delete the packages after they have been installed, thus this is a duplicate of ticket #680834, and I am marking it as such.