Remove obscure keyboard layouts from Greeter
The list of keyboard layout presented by the Greeter includes a lot of duplicate and obscure layouts, for example “Czech Slovak and German”.
The reason for this is that the /org/gnome/desktop/input-sources/show-all-sources dconf setting is enabled in the GDM session the greeter runs in, while it is disabled in a “normal” GNOME session.
For more information about this setting, see: https://blogs.gnome.org/mclasen/2012/09/21/input-sources-in-gnome/
I propose that we set this setting to false to get a more sensible list of keyboard layouts.