[SPARK-3415] [PySpark] removes SerializingAdapter code

This code removes the SerializingAdapter code that was copied from PiCloud
One final naming nit: so far, the classes named `*TestCase` are used for factoring out common setup / teardown code (such as `unittest.TestCase`, `PySparkTestCase`, etc.), while the classes with the actual tests have been called `[ComponentName]Tests` or `Test[ComponentName]`.  Therefore, I'd prefer to call this `CloudPickleTests`.

Also, since this is just testing CloudPickle without using any PySpark features, it should extend `unittest.TestCase` instead of `PySparkTestCase` so that we don't run a setup / teardown method for a SparkContext that we never use.