Remove unnecessary Build-Depends: gnome-themes-extra-data

Jeremy Bícharequested to merge
wip/bye-gnome-themes-extra-data into debian/latest

This Build-Depends was added in 2019 and appears to no longer be needed

Also, the Debian GNOME team is proposing to remove the gnome-themes-extra source package in https://bugs.debian.org/1112387

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