garbage-collection guide: noop read_record

[reynir]

The following read_record function doesn't do anything useful: https://github.com/ocaml/ocaml.org/blob/f4b7302d02d51b1f92796b9060e66d8977e309f8/data/tutorials/guides/1wf_05_garbage_collection.md?plain=1#L178-L181 The name and addr fields are copied into bytes, and the data is read into those bytes. However, the bytes are just thrown away and does nothing useful. Furthermore, nothing is done to ensure all is read.