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.

src/include/nodes/parsenodes.h

@@ -2139,7 +2139,6 @@ typedef struct Constraint
 	Node	   *raw_expr;		/* expr, as untransformed parse tree */
 	char	   *cooked_expr;	/* expr, as nodeToString representation */
 	char		generated_when; /* ALWAYS or BY DEFAULT */
-	char		generated_kind; /* currently always STORED */
 
 	/* Fields used for unique constraints (UNIQUE and PRIMARY KEY): */
 	List	   *keys;			/* String nodes naming referenced key

src/include/nodes/primnodes.h

@@ -410,8 +410,6 @@ typedef struct SubscriptingRef
 	List	   *reflowerindexpr;	/* expressions that evaluate to lower
 									 * container indexes, or NIL for single
 									 * container element */
-	List	   *refindexprslice;	/* whether or not related indexpr from
-									 * reflowerindexpr is a slice */
 	Expr	   *refexpr;		/* the expression that evaluates to a
 								 * container value */