[SPARK-4424] Remove spark.driver.allowMultipleContexts override in tests

This patch removes `spark.driver.allowMultipleContexts=true` from our test configuration. The multiple SparkContexts check was originally disabled because certain tests suites in SQL needed to create multiple contexts. As far as I know, this configuration change is no longer necessary, so we should remove it in order to make it easier to find test cleanup bugs.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #9865 from JoshRosen/SPARK-4424.