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

pkgs/servers/roundcube/default.nix

@@ -18,6 +18,8 @@ stdenv.mkDerivation rec {
     cp -r * $out/
     ln -sf /etc/roundcube/config.inc.php $out/config/config.inc.php
     rm -rf $out/installer
+    # shut up updater
+    rm $out/composer.json-dist
   '';
 
   passthru.withPlugins = f: buildEnv {