### What changes were proposed in this pull request?
As discussed on the Jira ticket, this change clears the SQLContext._instantiatedContext class attribute when the SparkSession is stopped. That way, the attribute will be reset with a new, usable SQLContext when a new SparkSession is started.
### Why are the changes needed?
When the underlying SQLContext is instantiated for a SparkSession, the instance is saved as a class attribute and returned from subsequent calls to SQLContext.getOrCreate(). If the SparkContext is stopped and a new one started, the SQLContext class attribute is never cleared so any code which calls SQLContext.getOrCreate() will get a SQLContext with a reference to the old, unusable SparkContext.
A similar issue was identified and fixed for SparkSession in [SPARK-19055](https://issues.apache.org/jira/browse/SPARK-19055), but the fix did not change SQLContext as well. I ran into this because mllib still [uses](https://github.com/apache/spark/blob/master/python/pyspark/mllib/common.py#L105) SQLContext.getOrCreate() under the hood.
### Does this PR introduce any user-facing change?
No
### How was this patch tested?
A new test was added. I verified that the test fails without the included change.
Closes#27610 from afavaro/restart-sqlcontext.
Authored-by: Alex Favaro <alex.favaro@affirm.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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