[SPARK-18874][SQL] Fix 2.10 build after moving the subquery rules to optimization

## What changes were proposed in this pull request?
Commit https://github.com/apache/spark/commit/4ce970d71488c7de6025ef925f75b8b92a5a6a79 in accidentally broke the 2.10 build for Spark. This PR fixes this by simplifying the offending pattern match.

## How was this patch tested?
Existing tests.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #17288 from hvanhovell/SPARK-18874.