/etc/apt/auth.conf should have lines removed when application/ppas are

Bug Description:
For Purchase items now make use of the /etc/apt/auth.conf file however the file doesn't get lines removed when applications are.

Mvo has said this isn't a huge issue but isn't an elegant solution.

SCA Version:
Django 1.3
Python 2.6.5 (r265:79063, Apr 16 2010, 14:15:55) [GCC 4.4.3]
preflight 0.1
SCA 12.03; r594
openid 2.2.1
pkgme-service https://pkgme-service.canonical.com/pkgme/api/1.0 {'version': 'r78'}

OS:
64bit Precise

USC Version:
5.1.12

Steps To Reproduce:
1. Install a for purchase app
2. Check the /etc/apt/auth.conf file
3. Now remove the app
4. Check the /etc/apt/auth.conf file
5. Now revome the repository if it hasn't been
6. Check the /etc/apt/auth.conf file

Expected Results:
The info connected to a repo/ppa should be reomoved when a repo/ppa is

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: software-center 5.1.12
ProcVersionSignature: Ubuntu 3.2.0-18.29-generic 3.2.9
Uname: Linux 3.2.0-18-generic x86_64
ApportVersion: 1.94.1-0ubuntu2
Architecture: amd64
Date: Thu Mar 15 13:46:10 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Beta amd64 (20120228)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_GB:en
 TERM=xterm
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: software-center
UpgradeStatus: No upgrade log present (probably fresh install)