Generate Vodafone link test sequence nos in proxy

Previously the Celery task to trigger a link test had to know about
the special case of a sequence number for Vodafone. Since we're about
to change the client to perform multiple tests it makes sense to give
it the knowledge of how to generate number itself.

Note that we have to import the db inline to avoid a circular import,
since this module is itself imported by app/__init__.py.

Other invocations of the Vodafone client use stored sequence numbers
from the DB, which are called "message numbers" in that context. Since
the two use cases are very different (even the names are different!),
having them in two places shouldn't cause any confusion.

app/celery/broadcast_message_tasks.py

@@ -2,9 +2,8 @@ import uuid
 from datetime import datetime
 
 from flask import current_app
-from sqlalchemy.schema import Sequence
 
-from app import cbc_proxy_client, db, notify_celery, zendesk_client
+from app import cbc_proxy_client, notify_celery, zendesk_client
 from app.clients.cbc_proxy import CBCProxyRetryableException
 from app.config import QueueNames
 from app.dao.broadcast_message_dao import (
@@ -252,11 +251,6 @@ def send_broadcast_provider_message(self, broadcast_event_id, provider):
 @notify_celery.task(name='trigger-link-test')
 def trigger_link_test(provider):
     identifier = str(uuid.uuid4())
-    formatted_seq_number = None
-    if provider == BroadcastProvider.VODAFONE:
-        sequence = Sequence('broadcast_provider_message_number_seq')
-        sequential_number = db.session.connection().execute(sequence)
-        formatted_seq_number = format_sequential_number(sequential_number)
     message = f"Sending a link test to CBC proxy for provider {provider}. Identifier in payload is {identifier}"
     current_app.logger.info(message)
-    cbc_proxy_client.get_proxy(provider).send_link_test(identifier, formatted_seq_number)
+    cbc_proxy_client.get_proxy(provider).send_link_test(identifier)