SQL: Add rule to prune unused aggregations.

[gianm]

Sometimes, unused aggregators can end up sticking around in a plan. For example, two of the three aggregators in this query are unused and can be eliminated:

SELECT
  CASE 'foo'
  WHEN 'bar' THEN SUM(cnt)
  WHEN 'foo' THEN SUM(m1)
  WHEN 'baz' THEN SUM(m2)
  END
FROM foo

This patch does that. And yes, I did see a query like this in the wild.