Clean up unneeded JSON depends in kdevplatform_add_plugin
The moc process already rebuilds the plugin when the JSON file changes, consequently the additional parameter is not needed.
Relates to https://phabricator.kde.org/T14649
The moc process already rebuilds the plugin when the JSON file changes, consequently the additional parameter is not needed.
Relates to https://phabricator.kde.org/T14649