Remove lightdm-set-defaults and gdmflexiserver.

Remove lightdm-set-defaults and gdmflexiserver.

lightdm-set-defaults is not required since 1.8 as configuration can now be loaded from a lightdm.conf.d directory. This allows a file per setting to be written and better handles more than one thing (e.g. a packaging system) setting these values.

Any software that still relies on gdmflexiserver should have switched to using dm-tool or the D-Bus interface by now.

Drop the GDM migration in the Ubuntu packaging. All Ubuntu users upgrading to this version of LightDM will be using at least 12.04 which had LightDM installed by default. (Only upgrades back to the last LTS are required to work).

Approved by Robert Ancell, PS Jenkins bot.