Unable to log in when default DE that was already removed still being set as default DE for a user

[Amir]

If users install additional Desktop Environments then uninstall all the additional DE's, all user accounts that used any of the removed DEs during their last login session will be then unable to log into despite being recreated because there's no way to select alternative DE at login screen when you have a single DE installed so those accounts will be stuck with the DE that was already removed

Steps to reproduce:

1. Install any additional Desktop Environment eg lxde
2. Create a new user
3. Log into lxde using that user account
4. Log out and go back to your main account
5. Remove the new user account
6. Remove all additional DE completely and now you're left with your one primary DE
7. Recreate the new user that you just removed
8. Try log into it and watch lightDM kicks you back to login screen

Solutions:
1. Make lightdm fallback to primary DE when user's default DE no longer exist on system
2. Automatically remove config that stores user's default DE in /var/lib/AccountsService/users/ when removing user account

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: lightdm 1.6.0-0ubuntu3
ProcVersionSignature: Ubuntu 3.8.0-29.42-generic 3.8.13.5
Uname: Linux 3.8.0-29-generic i686
ApportVersion: 2.9.2-0ubuntu8.3
Architecture: i386
Date: Wed Aug 28 12:28:31 2013
InstallationDate: Installed on 2013-05-06 (114 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release i386 (20130424)
MarkForUpload: True
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)