Remove dubious warning message from SQL/JSON functions

There was a warning that FORMAT JSON has no effect on json/jsonb
types, which is true, but it's not clear why we should issue a warning
about it.  The SQL standard does not say anything about this, which
should generally govern the behavior here.  So remove it.

Discussion: https://www.postgresql.org/message-id/flat/dfec2cae-d17e-c508-6d16-c2dba82db486%40eisentraut.org

src/backend/parser/parse_expr.c

@@ -3240,12 +3240,7 @@ transformJsonValueExpr(ParseState *pstate, const char *constructName,
 					parser_errposition(pstate, ve->format->location));
 
 		if (exprtype == JSONOID || exprtype == JSONBOID)
-		{
 			format = JS_FORMAT_DEFAULT; /* do not format json[b] types */
-			ereport(WARNING,
-					errmsg("FORMAT JSON has no effect for json and jsonb types"),
-					parser_errposition(pstate, ve->format->location));
-		}
 		else
 			format = ve->format->format_type;
 	}

src/test/regress/expected/sqljson.out

@@ -84,9 +84,6 @@ ERROR:  JSON ENCODING clause is only allowed for bytea input type
 LINE 1: SELECT JSON_OBJECT('foo': NULL::int FORMAT JSON ENCODING UTF...
                                             ^
 SELECT JSON_OBJECT('foo': NULL::json FORMAT JSON);
-WARNING:  FORMAT JSON has no effect for json and jsonb types
-LINE 1: SELECT JSON_OBJECT('foo': NULL::json FORMAT JSON);
-                                             ^
   json_object   
 ----------------
  {"foo" : null}
@@ -97,9 +94,6 @@ ERROR:  JSON ENCODING clause is only allowed for bytea input type
 LINE 1: SELECT JSON_OBJECT('foo': NULL::json FORMAT JSON ENCODING UT...
                                              ^
 SELECT JSON_OBJECT('foo': NULL::jsonb FORMAT JSON);
-WARNING:  FORMAT JSON has no effect for json and jsonb types
-LINE 1: SELECT JSON_OBJECT('foo': NULL::jsonb FORMAT JSON);
-                                              ^
   json_object  
 ---------------
  {"foo": null}