Do not add bogus line to apt preference file on Debian Wheezy

On Debian Wheezy the APT manifests can't be used with purge_preferences enabled. 

When purge_preferences is enabled a bogus information line is added to the file rendering it invalid and preventing APT from installing any packages.

After some research in the mater I found the only way to make it work again is to empty or remove the file. This pull request is tested on several Debain Wheezy installations. Without the fix all systems will cease to install any packages.