Removed docs of deprecated SimpleTestCase warnings behavior.

Removed in Django 1.7 (4f6be9a0c43050500af598527e1453d27c5c5b85).

docs/topics/testing/tools.txt

@@ -625,7 +625,6 @@ Normal Python unit test classes extend a base class of
 A thin subclass of :class:`unittest.TestCase`, it extends it with some basic
 functionality like:
 
-* Saving and restoring the Python warning machinery state.
 * Some useful assertions like:
 
   * Checking that a callable :meth:`raises a certain exception