bpo-33549: Remove shim and deprecation warning to access DocumentLS.async. (GH-6924)

`obj.async` is now a syntax error, so the warning/shim is
quasi-unnecessary.

Lib/xml/dom/xmlbuilder.py

@@ -332,29 +332,10 @@ class DOMBuilderFilter:
 del NodeFilter
 
 
-class _AsyncDeprecatedProperty:
-    def warn(self, cls):
-        clsname = cls.__name__
-        warnings.warn(
-            "{cls}.async is deprecated; use {cls}.async_".format(cls=clsname),
-            DeprecationWarning)
-
-    def __get__(self, instance, cls):
-        self.warn(cls)
-        if instance is not None:
-            return instance.async_
-        return False
-
-    def __set__(self, instance, value):
-        self.warn(type(instance))
-        setattr(instance, 'async_', value)
-
-
 class DocumentLS:
     """Mixin to create documents that conform to the load/save spec."""
 
     async_ = False
-    locals()['async'] = _AsyncDeprecatedProperty()  # Avoid DeprecationWarning
 
     def _get_async(self):
         return False
@@ -384,9 +365,6 @@ class DocumentLS:
         return snode.toxml()
 
 
-del _AsyncDeprecatedProperty
-
-
 class DOMImplementationLS:
     MODE_SYNCHRONOUS = 1
     MODE_ASYNCHRONOUS = 2