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.

src/flask/helpers.py

@@ -10,7 +10,6 @@ from threading import RLock
 
 import werkzeug.utils
 from werkzeug.exceptions import NotFound
-from werkzeug.local import ContextVar
 from werkzeug.routing import BuildError
 from werkzeug.urls import url_quote
 
@@ -800,26 +799,3 @@ def is_ip(value: str) -> bool:
             return True
 
     return False
-
-
-def async_to_sync(func: t.Callable[..., t.Coroutine]) -> t.Callable[..., t.Any]:
-    """Return a sync function that will run the coroutine function *func*.
-
-    This can be used as so
-
-        result = async_to_async(func)(*args, **kwargs)
-    """
-    try:
-        from asgiref.sync import async_to_sync as asgiref_async_to_sync
-    except ImportError:
-        raise RuntimeError(
-            "Install Flask with the 'async' extra in order to use async views."
-        )
-
-    # Check that Werkzeug isn't using its fallback ContextVar class.
-    if ContextVar.__module__ == "werkzeug.local":
-        raise RuntimeError(
-            "Async cannot be used with this combination of Python & Greenlet versions."
-        )
-
-    return asgiref_async_to_sync(func)