[SQL][MINOR] Simplify data source predicate filter translation.

## What changes were proposed in this pull request?
This is a small patch to rewrite the predicate filter translation in DataSourceStrategy. The original code used excessive functional constructs (e.g. unzip) and was very difficult to understand.

## How was this patch tested?
Should be covered by existing tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #13889 from rxin/simplify-predicate-filter.