[SPARK-10459] [SQL] Do not need to have ConvertToSafe for PythonUDF

JIRA: https://issues.apache.org/jira/browse/SPARK-10459

As mentioned in the JIRA, `PythonUDF` actually could process `UnsafeRow`.

Specially, the rows in `childResults` in `BatchPythonEvaluation` will be projected to a `MutableRow`. So I think we can enable `canProcessUnsafeRows` for `BatchPythonEvaluation` and get rid of redundant `ConvertToSafe`.

Author: Liang-Chi Hsieh <viirya@appier.com>

Closes #8616 from viirya/pyudf-unsafe.