nixos/rootston: Remove the module and the package (#48905)

Rootston is just a reference compositor so it doesn't make that much
sense to have a module for it. Upstream doesn't really like it as well:

"Rootston will never be intended for downstream packages, it's an
internal thing we use for testing." - SirCmpwn [0]

Removing the package and the module shouldn't cause much problems
because it was marked as broken until
886131c2. If required the package can
still be accessed via wlroots.bin (could be useful for testing
purposes).

[0]: https://github.com/NixOS/nixpkgs/issues/38344#issuecomment-378449256