[SPARK-50676][SQL] Remove unused `private lazy val mapValueContainsNu…

…ll` from `ElementAt`

### What changes were proposed in this pull request?
SPARK-33460 added `mapValueContainsNull` to `ElementAt`, but after SPARK-40066, this private lazy val is no longer used, so this pr removes it from `ElementAt`.

### Why are the changes needed?
Cleanup unused code.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Pass GitHub Actions

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #49300 from LuciferYang/SPARK-50676.

Authored-by: yangjie01 <yangjie01@baidu.com>
Signed-off-by: yangjie01 <yangjie01@baidu.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala

@@ -2609,9 +2609,6 @@ case class ElementAt(
 
   @transient private lazy val mapKeyType = left.dataType.asInstanceOf[MapType].keyType
 
-  @transient private lazy val mapValueContainsNull =
-    left.dataType.asInstanceOf[MapType].valueContainsNull
-
   @transient private lazy val arrayElementNullable =
     left.dataType.asInstanceOf[ArrayType].containsNull