Effectsmodel: Load KCM by path instead of searching for plugin by id

This way we do not need to embed json metadata in the plugins
and can consequently drop the metadata files entirely.

The resulting metadata object might still be invalid, however KCoreAddons
can handle such cases. See https://phabricator.kde.org/T15094 regarding
create better KCoreAddons API for such cases.

src/kcmkwin/common/effectsmodel.cpp

@@ -623,12 +623,11 @@ QModelIndex EffectsModel::findByPluginId(const QString &pluginId) const
 
 static KCModule *loadBinaryConfig(const QString &configModule, QObject *parent)
 {
-    const KPluginMetaData metaData = KPluginMetaData::findPluginById(QStringLiteral("kwin/effects/configs/"), configModule);
-
-    if (!metaData.isValid()) {
+    if (configModule.isEmpty()) {
         return nullptr;
     }
 
+    const KPluginMetaData metaData(QStringLiteral("kwin/effects/configs/") + configModule);
     return KPluginFactory::instantiatePlugin<KCModule>(metaData, parent).plugin;
 }