[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