Remove the async helper method

It is better to encourage users to utilise the app ensure_sync method
(or the newely added async_to_sync method) so that any extensions that
alter these methods take affect throughout the users code.

With the helper method users code fix parts of their code to the
asgiref async_to_sync ignoring any extension changes.

tests/test_async.py

@@ -6,7 +6,6 @@ import pytest
 from flask import Blueprint
 from flask import Flask
 from flask import request
-from flask.helpers import async_to_sync
 
 pytest.importorskip("asgiref")
 
@@ -136,5 +135,6 @@ def test_async_before_after_request():
 
 @pytest.mark.skipif(sys.version_info >= (3, 7), reason="should only raise Python < 3.7")
 def test_async_runtime_error():
+    app = Flask(__name__)
     with pytest.raises(RuntimeError):
-        async_to_sync(None)
+        app.async_to_sync(None)