[SQL] remove a decimal case branch that has no effect at runtime

it generates warnings at compile time marmbrus

Author: Xiangrui Meng <meng@databricks.com>

Closes #3192 from mengxr/dtc-decimal and squashes the following commits:

955e9fb [Xiangrui Meng] remove a decimal case branch that has no effect

sql/core/src/main/scala/org/apache/spark/sql/types/util/DataTypeConversions.scala

@@ -133,7 +133,6 @@ protected[sql] object DataTypeConversions {
   def convertJavaToCatalyst(a: Any, dataType: DataType): Any = (a, dataType) match {
     case (obj, udt: UserDefinedType[_]) => ScalaReflection.convertToCatalyst(obj, udt) // Scala type
     case (d: java.math.BigDecimal, _) => Decimal(BigDecimal(d))
-    case (d: java.math.BigDecimal, _) => BigDecimal(d)
     case (other, _) => other
   }