Deprecate service branding column

We want to drop this column. First we have to stop using it anywhere.

Needs to be made nullable so we can stop writing to it.

tests/app/db.py

@@ -78,7 +78,6 @@ def create_service(
         prefix_sms=True,
         message_limit=1000,
         organisation_type='central',
-        branding=None
 ):
     service = Service(
         name=service_name,
@@ -88,7 +87,6 @@ def create_service(
         created_by=user or create_user(email='{}@digital.cabinet-office.gov.uk'.format(uuid.uuid4())),
         prefix_sms=prefix_sms,
         organisation_type=organisation_type,
-        branding=branding
     )
 
     dao_create_service(service, service.created_by, service_id, service_permissions=service_permissions)