[SPARK-20304][SQL] AssertNotNull should not include path in string representation

## What changes were proposed in this pull request?
AssertNotNull's toString/simpleString dumps the entire walkedTypePath. walkedTypePath is used for error message reporting and shouldn't be part of the output.

## How was this patch tested?
Manually tested.

Author: Reynold Xin <rxin@databricks.com>

Closes #17616 from rxin/SPARK-20304.