Revert SQL/JSON features

The reverts the following and makes some associated cleanups:

    commit f79b803: Common SQL/JSON clauses
    commit f4fb45d: SQL/JSON constructors
    commit 5f0adec: Make STRING an unreserved_keyword.
    commit 33a3776: IS JSON predicate
    commit 1a36bc9: SQL/JSON query functions
    commit 606948b: SQL JSON functions
    commit 49082c2: RETURNING clause for JSON() and JSON_SCALAR()
    commit 4e34747: JSON_TABLE
    commit fadb48b: PLAN clauses for JSON_TABLE
    commit 2ef6f11: Reduce running time of jsonb_sqljson test
    commit 14d3f24: Further improve jsonb_sqljson parallel test
    commit a6baa4b: Documentation for SQL/JSON features
    commit b46bcf7: Improve readability of SQL/JSON documentation.
    commit 112fdb3: Fix finalization for json_objectagg and friends
    commit fcdb35c: Fix transformJsonBehavior
    commit 4cd8717: Improve a couple of sql/json error messages
    commit f7a605f: Small cleanups in SQL/JSON code
    commit 9c3d25e: Fix JSON_OBJECTAGG uniquefying bug
    commit a79153b: Claim SQL standard compliance for SQL/JSON features
    commit a1e7616: Rework SQL/JSON documentation
    commit 8d9f963: Fix errors in copyfuncs/equalfuncs support for JSON node types.
    commit 3c633f3: Only allow returning string types or bytea from json_serialize
    commit 67b2670: expression eval: Fix EEOP_JSON_CONSTRUCTOR and EEOP_JSONEXPR size.

The release notes are also adjusted.

Backpatch to release 15.

Discussion: https://postgr.es/m/40d2c882-bcac-19a9-754d-4299e1d87ac7@postgresql.org

Author: adunstan

doc/src/sgml/func.sgml

@@ -157,7 +157,6 @@ INTERVAL
 INTO
 IS
 JOIN
-JSON
 JSON_ARRAY
 JSON_ARRAYAGG
 JSON_EXISTS

doc/src/sgml/keywords/sql2016-02-reserved.txt

@@ -527,28 +527,28 @@ T652	SQL-dynamic statements in SQL routines			NO
 T653	SQL-schema statements in external routines			YES	
 T654	SQL-dynamic statements in external routines			NO	
 T655	Cyclically dependent routines			YES	
-T811	Basic SQL/JSON constructor functions			YES	
-T812	SQL/JSON: JSON_OBJECTAGG			YES	
-T813	SQL/JSON: JSON_ARRAYAGG with ORDER BY			YES	
-T814	Colon in JSON_OBJECT or JSON_OBJECTAGG			YES	
-T821	Basic SQL/JSON query operators			YES	
-T822	SQL/JSON: IS JSON WITH UNIQUE KEYS predicate			YES	
-T823	SQL/JSON: PASSING clause			YES	
-T824	JSON_TABLE: specific PLAN clause			YES	
-T825	SQL/JSON: ON EMPTY and ON ERROR clauses			YES	
-T826	General value expression in ON ERROR or ON EMPTY clauses			YES	
-T827	JSON_TABLE: sibling NESTED COLUMNS clauses			YES	
-T828	JSON_QUERY			YES	
-T829	JSON_QUERY: array wrapper options			YES	
-T830	Enforcing unique keys in SQL/JSON constructor functions			YES	
+T811	Basic SQL/JSON constructor functions			NO	
+T812	SQL/JSON: JSON_OBJECTAGG			NO	
+T813	SQL/JSON: JSON_ARRAYAGG with ORDER BY			NO	
+T814	Colon in JSON_OBJECT or JSON_OBJECTAGG			NO	
+T821	Basic SQL/JSON query operators			NO	
+T822	SQL/JSON: IS JSON WITH UNIQUE KEYS predicate			NO	
+T823	SQL/JSON: PASSING clause			NO	
+T824	JSON_TABLE: specific PLAN clause			NO	
+T825	SQL/JSON: ON EMPTY and ON ERROR clauses			NO	
+T826	General value expression in ON ERROR or ON EMPTY clauses			NO	
+T827	JSON_TABLE: sibling NESTED COLUMNS clauses			NO	
+T828	JSON_QUERY			NO	
+T829	JSON_QUERY: array wrapper options			NO	
+T830	Enforcing unique keys in SQL/JSON constructor functions			NO	
 T831	SQL/JSON path language: strict mode			YES	
 T832	SQL/JSON path language: item method			YES	
 T833	SQL/JSON path language: multiple subscripts			YES	
 T834	SQL/JSON path language: wildcard member accessor			YES	
 T835	SQL/JSON path language: filter expressions			YES	
 T836	SQL/JSON path language: starts with predicate			YES	
 T837	SQL/JSON path language: regex_like predicate			YES	
-T838	JSON_TABLE: PLAN DEFAULT clause			YES	
+T838	JSON_TABLE: PLAN DEFAULT clause			NO	
 T839	Formatted cast of datetimes to/from character strings			NO	
 M001	Datalinks			NO	
 M002	Datalinks via SQL/CLI			NO	

src/backend/catalog/sql_features.txt

@@ -3851,13 +3851,7 @@ ExplainTargetRel(Plan *plan, Index rti, ExplainState *es)
 			break;
 		case T_TableFuncScan:
 			Assert(rte->rtekind == RTE_TABLEFUNC);
-			if (rte->tablefunc)
-				if (rte->tablefunc->functype == TFT_XMLTABLE)
-					objectname = "xmltable";
-				else			/* Must be TFT_JSON_TABLE */
-					objectname = "json_table";
-			else
-				objectname = NULL;
+			objectname = "xmltable";
 			objecttag = "Table Function Name";
 			break;
 		case T_ValuesScan: