articulated_object reader: fixing faulty memory subscription

• We cannot ensure that a component derives from the MemoryListener interface => raises warning due to

memoryNameSystem.subscribe(id, this, &Writer::updateKnownObjects);

in https://git.h2t.iar.kit.edu/sw/armarx/robot-api/-/blob/master/source/RobotAPI/libraries/armem_objects/client/articulated_object/Writer.cpp?ref_type=heads#L78

• Also, the implementation can be simplified. We don't actually need the caching mechanism (which can also introduce general caching problems). Instead, we query the data from memory.