Clean up unneeded JSON parameter in kcoreaddons_add_plugin
The moc process already rebuilds the plugin when the JSON file changes, consequently the additional parameter is not needed.
Task: https://phabricator.kde.org/T14649
@alexmerry I am wondering why there are JSON files lying around, but they do not contain all of the info the desktop files do. But I don't know anything about the usages of this framework.