Drop legacy flag USE_KPROCESS_FOR_KIOSLAVES
Used pre-KF5, missed to be cleaned-up in port of kioslave executable to QPluginLoader in unmaintained/kdelibs@2b6b88b4
Used pre-KF5, missed to be cleaned-up in port of kioslave executable to QPluginLoader in unmaintained/kdelibs@2b6b88b4