[2.1.x] Refs #27807 -- Removed docs for User.username_validator.

The new override functionality claimed in refs #21379 doesn't work.
Forwardport of 714fdbaa from stable/1.10.x.

docs/ref/contrib/auth.txt

@@ -38,8 +38,7 @@ Fields
             usernames. Although it wasn't a deliberate choice, Unicode
             characters have always been accepted when using Python 3. Django
             1.10 officially added Unicode support in usernames, keeping the
-            ASCII-only behavior on Python 2, with the option to customize the
-            behavior using :attr:`.User.username_validator`.
+            ASCII-only behavior on Python 2.
 
     .. attribute:: first_name
 
@@ -141,24 +140,6 @@ Attributes
         :attr:`~django.contrib.auth.models.User.is_authenticated` to this
         attribute.
 
-    .. attribute:: username_validator
-
-        Points to a validator instance used to validate usernames. Defaults to
-        :class:`validators.UnicodeUsernameValidator`.
-
-        To change the default username validator, you can subclass the ``User``
-        model and set this attribute to a different validator instance. For
-        example, to use ASCII usernames::
-
-            from django.contrib.auth.models import User
-            from django.contrib.auth.validators import ASCIIUsernameValidator
-
-            class CustomUser(User):
-                username_validator = ASCIIUsernameValidator()
-
-                class Meta:
-                    proxy = True  # If no new field is added.
-
 Methods
 -------
 

docs/releases/1.10.txt

@@ -60,12 +60,11 @@ wasn't a deliberate choice, Unicode characters have always been accepted when
 using Python 3.
 
 The username validator now explicitly accepts Unicode characters by
-default on Python 3 only. This default behavior can be overridden by changing
-the :attr:`~django.contrib.auth.models.User.username_validator` attribute of
-the ``User`` model, or to any proxy of that model, using either
+default on Python 3 only.
+
+Custom user models may use the new
 :class:`~django.contrib.auth.validators.ASCIIUsernameValidator` or
-:class:`~django.contrib.auth.validators.UnicodeUsernameValidator`. Custom user
-models may also use those validators.
+:class:`~django.contrib.auth.validators.UnicodeUsernameValidator`.
 
 Minor features
 --------------