Please remove all ltsp* blacklisting

[Impact]
Please remove all references to the ltsp-client, ltsp-client-core, ltspfs, and ltspfsd packages from the update-manager sources, as update-manager currently removes those packages on dist-upgrades, breaking LTSP installations.
Update-manager has seen a lot of changes and that blacklisting no longer exist in 14.04+, so we only need this as an SRU for 12.04.

Rationale:
Historically, the ltsp-client package had a preinst check that aborted its installation in normal systems (by checking for an /etc/ltsp_chroot file).

If someone managed to install it outside of an LTSP chroot, then dist-upgrades were broken there, and a workaround was introduced in update-manager 1:0.60 gutsy Tue, 24 Apr 2007 16:39:26 +0200.
This is mentioned in the update-manager changelog and in LP #109638.

But, since Ubuntu Precise, ltsp-client no longer has these restrictions (I'm an upstream LTSP developer) and it's even useful to have it installed in normal LTSP servers (see https://help.ubuntu.com/community/UbuntuLTSP/ltsp-pnp for more info).

Now the opposite problem arises: when there's need for `update-manager -dist-upgrade`, update-manager removes at least ltsp-client and ltspfsd, breaking LTSP installations.
See the attached update-manager screenshot.

Actual use case: a teacher is prompted for a normal update, where some packages need to be updated, and additionally some unrelated package is now obsolete and needs to be removed. In that case, update-manager starts a dist-upgrade (not to be confused with a release upgrade, it's normal maintenance, not e.g. upgrading from 12.04 to 14.04) and removes ltsp-client and ltspfsd. The LTSP server stops working and the teacher doesn't know why, he never even pressed the "expand" button in the update dialog to see that those packages were uninstalled.

[Test Case]
* Install LTSP client, for example with:
sudo apt-get install ltsp-client
* Just to make sure that your apt is in a consistent state, run:
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get purge --auto-remove
...we expect no output from these.
* Now run:
update-manager --dist-upgrade
See that update-manager prompts for ltsp-client and ltspfsd removal, mentioning they're no longer needed, as shown in the attached screenshot.

On the other hand, with the included patch, `update-manager --dist-upgrade` shouldn't mention that anything needs removal.

[Regression Potential]
Not much. In 14.04+ that blacklisting is already removed, ltsp isn't mentioned anywhere in the update-manager sources anymore.

And of course we'll be testing the package from precise-proposed in many installations (specifically, hundreds of Greek schools) before confirming that it's OK to copy it to -updates.