Reconcile nodes/*funcs.c with PostgreSQL 12 work.

Noah Misch <noah@leadboat.com>

Commit: 44982e7d09cf9b1a08fded7fb0ca9a60dae869a5
Author: Noah Misch <noah@leadboat.com>
Date: 2019-06-09T21:00:36Z
Releases: 12.0
Reconcile nodes/*funcs.c with PostgreSQL 12 work.

One would have needed out-of-tree code to observe the defects.  Remove
unreferenced fields instead of completing their support functions.
Since in-tree code can't reach _readIntoClause(), no catversion bump.

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