Get rid of ParseState.p_value_substitute; use a columnref hook instead.

I noticed that p_value_substitute, which is a single-purpose kluge I added
in 2002 (commit b0422b215), could be replaced by having domainAddConstraint
install a parser hook that looks for the name "value".  The parser hook
code only dates back to 2009, so it's not surprising that we had to kluge
this in 2002, but we can do it more cleanly now.

src/include/parser/parse_node.h

@@ -149,9 +149,6 @@ typedef Node *(*CoerceParamHook) (ParseState *pstate, Param *param,
  * p_locked_from_parent: true if parent query level applies FOR UPDATE/SHARE
  * to this subquery as a whole.
  *
- * p_value_substitute: replacement for VALUE references, if we're parsing
- * a domain CHECK constraint.
- *
  * p_hasAggs, p_hasWindowFuncs, etc: true if we've found any of the indicated
  * constructs in the query.
  *
@@ -184,7 +181,6 @@ struct ParseState
 	List	   *p_locking_clause;		/* raw FOR UPDATE/FOR SHARE info */
 	bool		p_locked_from_parent;	/* parent has marked this subquery
 										 * with FOR UPDATE/FOR SHARE */
-	Node	   *p_value_substitute;		/* what to replace VALUE with, if any */
 
 	/* Flags telling about things found in the query: */
 	bool		p_hasAggs;