Old autostarts are not cleaned up on upgrade

Because the autostart files installed by this package go into `/etc`, they're considered "conffiles" by dpkg. This means they are not automatically deleted when a new version of the package no longer carries them.

If we look on a system that had an older version of `pantheon-xsession-settings` installed, we can see there are two sets of autostarts in `/etc/xdg/autostart`. We have one set that are called `x-pantheon.desktop` and another that are `pantheon-x.desktop`.

This causes some problems where we have a lot of the gsd components loading twice.

This will obviously only affect people that have upgraded from an older version of the package (so it breaks the currently nonexistent bionic -> focal upgrade flow and anyone that was on an older daily build of 6.0). But, it will also cause problems in the future if we decide to stop shipping a particular autostart for some reason.

@tintou can you think of any smart/automatic way we might clean up old files in `/etc/` that used to belong to this package but don't anymore. Obviously we could write a `postinst` with a list of files to remove, but that requires someone to keep manually updating the file every time we remove/change an autostart.