planbuilder: Fix infinite recursion for subqueries

[dbussink]

The rewriting on v16 didn't consider the case where we already had an extract subquery. In that case we don't extract again, to avoid infinite recursion.

This does not affect v17 and later as this was fixed in the refactor in #12197.

Related Issue(s)

Fixes #13780

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[vitess-bot]

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