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.

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/39/commits

Related issues

Related to #16095 (closed)
Blocked by #16095 (closed)

Original created by @segfault on 17089 (Redmine)

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