In this PR I delete a method that breaks type inference for aggregators (only in the REPL)

The error when this method is present is:
```
<console>:38: error: missing parameter type for expanded function ((x$2) => x$2._2)
              ds.groupBy(_._1).agg(sum(_._2), sum(_._3)).collect()
```

Author: Michael Armbrust <michael@databricks.com>

Closes #9870 from marmbrus/dataset-repl-agg.