Removed available_apps on TestCase subclasses.

TestCase subclasses are wrapped in a transaction that prevents any data from
being persisted between test runs andi thus don't require limiting the tables
to be flushed to a subset of available apps like TransactionTestCase subclasses
do.

tests/multiple_database/tests.py

@@ -1794,11 +1794,6 @@ class SyncOnlyDefaultDatabaseRouter(object):
 
 class MigrateTestCase(TestCase):
 
-    available_apps = [
-        'multiple_database',
-        'django.contrib.auth',
-        'django.contrib.contenttypes'
-    ]
     multi_db = True
 
     def test_migrate_to_other_database(self):