[SPARK-38100][SQL] Remove unused private method in Decimal

### What changes were proposed in this pull request?
There is an unused `private` method `overflowException` in `org.apache.spark.sql.types.Decimal`, this method add by SPARK-28741 and  the relevant invocations are replaced by `QueryExecutionErrors.castingCauseOverflowError` directly after SPARK-35060. So this pr remove this unused method.

### Why are the changes needed?
Remove unused method.

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

### How was this patch tested?
Pass GA

Closes apache#35392 from LuciferYang/SPARK-38100.

Authored-by: yangjie01 <yangjie01@baidu.com>
Signed-off-by: huaxingao <huaxin_gao@apple.com>
(cherry picked from commit 7a613ec)
Signed-off-by: huaxingao <huaxin_gao@apple.com>
(cherry picked from commit 4cd3fd3)

sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala

@@ -251,9 +251,6 @@ final class Decimal extends Ordered[Decimal] with Serializable {
 
   def toByte: Byte = toLong.toByte
 
-  private def overflowException(dataType: String) =
-    throw QueryExecutionErrors.castingCauseOverflowError(this, dataType)
-
   /**
    * @return the Byte value that is equal to the rounded decimal.
    * @throws ArithmeticException if the decimal is too big to fit in Byte type.