[SPARK-3415] [PySpark] removes SerializingAdapter code

This code removes the SerializingAdapter code that was copied from PiCloud
Also, `test_cloudpickle` isn't a very descriptive name; it will be hard for people that come along and read this later to figure out what this is supposed to be testing.  A better name would be `test_pickling_file_handles` (and maybe add a comment saying that it's a regression test for SPARK-3415).