[SPARK-15037][HOTFIX] Don't create 2 SparkSessions in constructor

## What changes were proposed in this pull request?

After #12907 `TestSparkSession` creates a spark session in one of the constructors just to get the `SparkContext` from it. This ends up creating 2 `SparkSession`s from one call, which is definitely not what we want.

## How was this patch tested?

Jenkins.

Author: Andrew Or <andrew@databricks.com>

Closes #13031 from andrewor14/sql-test.

sql/core/src/test/scala/org/apache/spark/sql/test/TestSQLContext.scala

@@ -31,17 +31,7 @@ private[sql] class TestSparkSession(sc: SparkContext) extends SparkSession(sc) {
   }
 
   def this() {
-    this {
-      val conf = new SparkConf()
-      conf.set("spark.sql.testkey", "true")
-
-      val spark = SparkSession.builder
-        .master("local[2]")
-        .appName("test-sql-context")
-        .config(conf)
-        .getOrCreate()
-      spark.sparkContext
-    }
+    this(new SparkConf)
   }
 
   @transient