lightdm/lightdm.conf: syncronized the configuration file with the one of the latest lightdm.

A recent LightDM upgrade made the greeter unstartable.
While I found the culprit here ([SeatDefaults]'s greeter-session), it's a good measure to syncronize
the whole configuration file with the one from upstream.
debian/: ensure that the infamous greeter-session setting is commented out when upgrading the package.

Brave users can re-enable it anytime: the next semplice-openbox-default-settings upgrade will leave
everything as it is.

(also, moved postrm to prerm)
6.1.1