[libcalamares] Drop INTERFACES again

- The compile failure came from bad #include paths, so restoring
   this interface declaration wasn't a fix.
 - Reported to cause runtime failures on both KaOS and Manjaro.

src/libcalamares/utils/PluginFactory.h

@@ -101,7 +101,4 @@ public:
 #define CALAMARES_PLUGIN_FACTORY_DEFINITION( name, pluginRegistrations ) \
     K_PLUGIN_FACTORY_DEFINITION_WITH_BASEFACTORY( name, CalamaresPluginFactory, pluginRegistrations )
 
-
-Q_DECLARE_INTERFACE( CalamaresPluginFactory, CalamaresPluginFactory_iid )
-
 #endif