It was basically just a `environment.systemPackages` synonym,
only GNOME used it, and it was stretching the responsibilities
of the flatpak module too far.
It also makes it cleaner to avoid installing the program
using GNOME module’s `excludePackages` option.
Partially reverts: https://github.com/NixOS/nixpkgs/pull/101516
Fixes: https://github.com/NixOS/nixpkgs/issues/110310
Gnome doesn't work at least since I started using NixOS half a year
ago, let's not give wrong impressions to newcomers. Packaging gnome3
is still something on horizon.
