Remove the django_db mark from TransactionTestCase classes

pytest-django's documentation indicates that it isn't necessary, and
it turns out in Django 2.0+ that this double application of the
transaction machinations was causing the Django ContentType table to
lose its items.

awx/main/tests/functional/test_rbac_user.py

@@ -7,7 +7,6 @@ from awx.main.access import UserAccess, RoleAccess, TeamAccess
 from awx.main.models import User, Organization, Inventory
 
 
-@pytest.mark.django_db
 class TestSysAuditorTransactional(TransactionTestCase):
     def rando(self):
         return User.objects.create(username='rando', password='rando', email='rando@com.com')