gdm: fix postinst-code to unbreak first boot issues

Exiting postinst code leaves systemd.bbclass offline postinst code unexecuted,
causes postinst not to finish at first boot (gdm.service is not aliased to
display-manager.service -> started much too early) and is not neccesary:
language preparation can be done offline.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>