fix: Drop the KNSCore EngineBase cache method
KNewStuff API Change (frameworks/knewstuff!332 (diffs)) deprecated the cache method on KNSCore::EngineBase. Here the references to the cache method are deleted, relying on the KNSCore::Engine handling of cache. (An alternative could be to activate the preprocessor flag KNEWSTUFFCORE_DEPRECATED on KNewStuff.)