Also allowed a non-overridden setting to be deleted

Refs #20032, #18824. Thanks ztorstri at gmail.com for the report.

docs/topics/testing/tools.txt

@@ -1193,6 +1193,11 @@ have been overridden, like this::
         del settings.LOGIN_URL
         ...
 
+.. versionchanged:: 1.7
+
+    Previously, you could only simulate the deletion of a setting which was
+    explicitely overridden.
+
 When overriding settings, make sure to handle the cases in which your app's
 code uses a cache or similar feature that retains state even if the setting is
 changed. Django provides the :data:`django.test.signals.setting_changed`