[SPARK-21941] Stop storing unused attemptId in SQLTaskMetrics

## What changes were proposed in this pull request?

In a driver heap dump containing 390,105 instances of SQLTaskMetrics this
would have saved me approximately 3.2MB of memory.

Since we're not getting any benefit from storing this unused value, let's
eliminate it until a future PR makes use of it.

## How was this patch tested?

Existing unit tests

Author: Andrew Ash <andrew@andrewash.com>

Closes #19153 from ash211/aash/trim-sql-listener.