MDEV-11439 No data type JSON, but CAST(something AS JSON) pretends to work

remove CAST(... AS JSON) from the grammar for 10.2.4

mysql-test/r/func_json.result

@@ -402,15 +402,6 @@ json_object("a", json_object("b", "abcd"))
 select json_object("a", '{"b": "abcd"}');
 json_object("a", '{"b": "abcd"}')
 {"a": "{\"b\": \"abcd\"}"}
-select json_object("a", cast('{"b": "abcd"}' as json));
-json_object("a", cast('{"b": "abcd"}' as json))
-{"a": {"b": "abcd"}}
-select cast(NULL AS JSON);
-cast(NULL AS JSON)
-NULL
-select json_depth(cast(NULL as JSON));
-json_depth(cast(NULL as JSON))
-NULL
 select json_depth('[[], {}]');
 json_depth('[[], {}]')
 2

mysql-test/t/func_json.test

@@ -161,10 +161,10 @@ select json_unquote('abc');
 
 select json_object("a", json_object("b", "abcd"));
 select json_object("a", '{"b": "abcd"}');
-select json_object("a", cast('{"b": "abcd"}' as json));
+#select json_object("a", cast('{"b": "abcd"}' as json));
 
-select cast(NULL AS JSON);
-select json_depth(cast(NULL as JSON));
+#select cast(NULL AS JSON);
+#select json_depth(cast(NULL as JSON));
 select json_depth('[[], {}]');
 select json_depth('[[[1,2,3],"s"], {}, []]');
 select json_depth('[10, {"a": 20}]');

sql/sql_yacc.yy

@@ -10714,7 +10714,6 @@ cast_type:
           }
         | cast_type_numeric  { $$= $1; Lex->charset= NULL; }
         | cast_type_temporal { $$= $1; Lex->charset= NULL; }
-        | JSON_SYM           { $$.set(ITEM_CAST_JSON); }
         ;
 
 cast_type_numeric: