roundcube: remove composer.json-dist to prevent roundcube from trying to update composer.json
roundcube: remove composer.json-dist to prevent roundcube from trying to update composer.json

this removes the warning

```
WARNING: unable to update composer.json!
```

since it might be confusing people, as nixOS already handles dependency updates

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs