[1.6.x] Removed thread customizations of six which are now built-in.

Backport of 7ef81b5cdd from master

django/utils/six.py

@@ -786,7 +786,3 @@ def assertRaisesRegex(self, *args, **kwargs):
 
 def assertRegex(self, *args, **kwargs):
     return getattr(self, _assertRegex)(*args, **kwargs)
-
-
-add_move(MovedModule("_dummy_thread", "dummy_thread"))
-add_move(MovedModule("_thread", "thread"))

docs/topics/python3.txt

@@ -398,7 +398,3 @@ extras.
     This replaces ``testcase.assertRegexpMatches`` on Python 2, and
     ``testcase.assertRegex`` on Python 3. ``assertRegexpMatches`` still
     exists in current Python 3 versions, but issues a warning.
-
-
-In addition to six' defaults moves, Django's version provides ``thread`` as
-``_thread`` and ``dummy_thread`` as ``_dummy_thread``.