Fix PySpark tests + remove sc.clearFiles/clearJars

The reason why PySpark tests failed was because this PR previously introduced a default
parameter to a few methods in SparkContext, and this was not understood by the Py4j
conversion of JavaSparkContext, since Java does not have default parameters. The fix
gets rid of the use of default parameters, which also simplifies the logic of triggering
SparkListenerEnvironmentUpdate events.

This commit also deprecates sc.clearFiles and sc.clearJars, since they achieve little
beyond deleting a few map entries when the SparkContext is already terminating anyway.