[SPARK-19447] Remove remaining references to generated rows metric

## What changes were proposed in this pull request?

https://github.com/apache/spark/commit/b486ffc86d8ad6c303321dcf8514afee723f61f8 left behind references to "number of generated rows" metrics, that should have been removed.

## How was this patch tested?

Existing unit tests.

Author: Ala Luszczak <ala@databricks.com>

Closes #17939 from ala/SPARK-19447-fix.