testing/gnome-themes-extra: move from community and unmaintain

This is just legacy GTK2 theming, not present anymore in anything we should care about[1]. I would have gone as far as dropping it, but arc-theme, materia-gtk-theme and mint-themes from testing depend on it, so I guess it should be up to authors to take care of it :)

Relates !39229 (merged)

[1] https://gitlab.gnome.org/Teams/Design/initiatives/-/issues/107