Remove branding attribute from service model

To correspond with us dropping this column from the database.

Remove the attribute from the model gives us more confidence that it’s
not being used (because it will raise exceptions in any tests that refer
to it).

app/notify_client/models.py

@@ -270,7 +270,6 @@ class Service(dict):
 
     ALLOWED_PROPERTIES = {
         'active',
-        'branding',
         'dvla_organisation',
         'email_branding',
         'email_from',
@@ -293,7 +292,7 @@ class Service(dict):
     def __getattr__(self, attr):
         if attr in self.ALLOWED_PROPERTIES:
             return self[attr]
-        raise AttributeError
+        raise AttributeError('`{}` is not a service attribute'.format(attr))
 
     @property
     def trial_mode(self):