[SPARK-4904] [SQL] Remove the unnecessary code change in Generic UDF

Since #3429 has been merged, the bug of wrapping to Writable for HiveGenericUDF is resolved, we can safely remove the foldable checking in `HiveGenericUdf.eval`, which discussed in #2802.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #3745 from chenghao-intel/generic_udf and squashes the following commits:

622ad03 [Cheng Hao] Remove the unnecessary code change in Generic UDF

sql/hive/src/main/scala/org/apache/spark/sql/hive/hiveUdfs.scala

@@ -158,11 +158,6 @@ private[hive] case class HiveGenericUdf(funcWrapper: HiveFunctionWrapper, childr
   override def foldable =
     isUDFDeterministic && returnInspector.isInstanceOf[ConstantObjectInspector]
 
-  @transient
-  protected def constantReturnValue = unwrap(
-    returnInspector.asInstanceOf[ConstantObjectInspector].getWritableConstantValue(),
-    returnInspector)
-  
   @transient
   protected lazy val deferedObjects =
     argumentInspectors.map(new DeferredObjectAdapter(_)).toArray[DeferredObject]
@@ -171,7 +166,6 @@ private[hive] case class HiveGenericUdf(funcWrapper: HiveFunctionWrapper, childr
 
   override def eval(input: Row): Any = {
     returnInspector // Make sure initialized.
-    if(foldable) return constantReturnValue
 
     var i = 0
     while (i < children.length) {