Removing package leaves motd script in place, renders incorrect motd

[Andreas Hasenack]

Removing the ubuntu-advantage-tools package (not purging: removing) will leave the /etc/update-motd.d/99-esm script in place. Accordingly, it will run at login time, but since it calls out to the ubuntu-advantage script that no longer exists, it fails.

This is the sequence of events:
- ubuntu-advantage-tools is installed
- sudo ubuntu-advantage enable-esm with a valid token is run
- logout and login, confirming that the MOTD says something like "This Ubuntu 12.04 system is configured to receive extended security updates"
- remove (not purge) the ubuntu-advantage package. This will leave the sources.list.d/ file with the ESM repository in place and working, but remove the ubuntu-advantage script
- logout and login. The MOTD will now say "This Ubuntu 12.04 LTS system is past its End of Life", even though the ESM repository is still enabled and functioning.

[Lucas Albuquerque Medeiros de Moura (lamoura)]

We no longer ship the 99-esm file, so we shouldn't be affected by this issue anymore.
Additionally, I have double checked the new update-motd scripts we deliver and I can confirm they will not break the system if the user just removes the packages, since we have a check that prevent invalid files from being read, for example, this is the output of our 88-esm-announce script:

```
#!/bin/sh
stamp="/var/lib/ubuntu-advantage/messages/motd-esm-announce"

[ ! -r "$stamp" ] || cat "$stamp"
```

Because of that, I am marking this bug as invalid for the current version of UA.