[MINOR][SQL][TEST] no uncache table in joinsuite test

## What changes were proposed in this pull request?

At present, in test("broadcasted hash outer join operator selection") case,  set the testData2 to _CACHE TABLE_, but no _uncache table_ testData2. It can make people confused.
In addition, in the joinsuite test cases, clear the cache table of work by SharedSQLContext _spark.sharedState.cacheManager.clearCache_ to do, so we do not need to uncache table
let's fix it. thanks.

## How was this patch tested?
Existing test cases.

Author: caoxuewen <[email protected]>

Closes apache#18914 from heary-cao/uncache_table.

Author: heary-cao

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

@@ -126,7 +126,6 @@ class JoinSuite extends QueryTest with SharedSQLContext {
       ("SELECT * FROM testData join testData2 ON key = a where key = 2",
         classOf[BroadcastHashJoinExec])
     ).foreach(assertJoin)
-    sql("UNCACHE TABLE testData")
   }
 
   test("broadcasted hash outer join operator selection") {
@@ -141,7 +140,6 @@ class JoinSuite extends QueryTest with SharedSQLContext {
       ("SELECT * FROM testData right join testData2 ON key = a and key = 2",
         classOf[BroadcastHashJoinExec])
     ).foreach(assertJoin)
-    sql("UNCACHE TABLE testData")
   }
 
   test("multiple-key equi-join is hash-join") {
@@ -476,7 +474,6 @@ class JoinSuite extends QueryTest with SharedSQLContext {
       ).foreach(assertJoin)
     }
 
-    sql("UNCACHE TABLE testData")
   }
 
   test("cross join with broadcast") {
@@ -565,7 +562,6 @@ class JoinSuite extends QueryTest with SharedSQLContext {
           Row("2", 3, 2) :: Nil)
     }
 
-    sql("UNCACHE TABLE testData")
   }
 
   test("left semi join") {