Back out the EXISTS-to-IN optimization. It slows things down rather than

Back out the EXISTS-to-IN optimization.  It slows things down rather than
speeds them up depending on the query.  And (see
[forum:/forumpost/8692d94725|forum post 8692d94725]) it sometimes results in
an incorrect answer.  We may come back and revisit this optimization later,
but for now it seems best just to disable it.

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