remove async_sequence return

The grammar isn't wrong but it's not valid. It's covered in invalid/idl instead.

Author: saschanaz

test/syntax/baseline/async-sequence.json

@@ -124,75 +124,6 @@
         "extAttrs": [],
         "partial": false
     },
-    {
-        "type": "interface",
-        "name": "asyncIterableReturn",
-        "inheritance": null,
-        "members": [
-            {
-                "type": "operation",
-                "name": "stream",
-                "idlType": {
-                    "type": "return-type",
-                    "extAttrs": [],
-                    "generic": "async_sequence",
-                    "nullable": false,
-                    "union": false,
-                    "idlType": [
-                        {
-                            "type": "return-type",
-                            "extAttrs": [],
-                            "generic": "",
-                            "nullable": false,
-                            "union": false,
-                            "idlType": "short"
-                        }
-                    ]
-                },
-                "arguments": [
-                    {
-                        "type": "argument",
-                        "name": "foo",
-                        "extAttrs": [],
-                        "idlType": {
-                            "type": "argument-type",
-                            "extAttrs": [],
-                            "generic": "async_sequence",
-                            "nullable": false,
-                            "union": false,
-                            "idlType": [
-                                {
-                                    "type": "argument-type",
-                                    "extAttrs": [],
-                                    "generic": "",
-                                    "nullable": false,
-                                    "union": false,
-                                    "idlType": "short"
-                                }
-                            ]
-                        },
-                        "default": null,
-                        "optional": false,
-                        "variadic": false
-                    }
-                ],
-                "extAttrs": [],
-                "special": ""
-            }
-        ],
-        "extAttrs": [
-            {
-                "type": "extended-attribute",
-                "name": "Exposed",
-                "rhs": {
-                    "type": "identifier",
-                    "value": "Window"
-                },
-                "arguments": []
-            }
-        ],
-        "partial": false
-    },
     {
         "type": "eof",
         "value": ""

test/syntax/idl/async-sequence.webidl

@@ -5,8 +5,3 @@ interface Canvas {
 interface I {
   Promise<undefined> f1(async_sequence<[XAttr] float> arg);
 };
-
-[Exposed=Window]
-interface asyncIterableReturn {
-  async_sequence<short> stream(async_sequence<short> foo);
-};