Defer deleting subqueries in the compound-SELECT code generator until the

Defer deleting subqueries in the compound-SELECT code generator until the
end of code generation, in order to avoid deleting expressions out from under
the aggregation function sanity checking assert()s that occur near the
end of SELECT code generation.  This fixes the assertion fault described by
[forum:/forumpost/cfcb4b461d|forum post cfcb4b461d].

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