Purge /etc/apt/apt.conf

/etc/apt/apt.conf gets created during the installation process, either
via d-i or labs vmbuilder. It is no longer needed after the install is
over and given it is unpuppetized it causes various problems during
migrations

This was found during RT: 6133

Change-Id: I8c0031851f2f3cf5aa0b558a564e31de70cfbfc8