Removed superfluous models.py files.

Added comments in the three empty models.py files that are still needed.

Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.

django/contrib/admindocs/models.py

@@ -1 +0,0 @@
-# Empty models.py to allow for specifying admindocs as a test label.

django/contrib/formtools/models.py

@@ -1 +1,2 @@
-""" models.py (even empty) currently required by the runtests.py to enable unit tests """
+# This file is required to pretend formtools has models.
+# Otherwise test models cannot be registered.

django/contrib/messages/models.py

@@ -1 +0,0 @@
-# Models module required so tests are discovered.

django/contrib/sitemaps/models.py

@@ -1 +1,2 @@
-# This file intentionally left blank
+# This file is required to pretend sitemaps has models.
+# Otherwise test models cannot be registered.