Log activity on all periodic Celery tasks

As stated in the comment, this would have been helpful during an
incident to give further reassurance that a task had at least
started running - at the time the only evidence for this was the
Cronitor dashboard itself, which we don't often look at.

I've removed other, equivalent "starting" logs, but kept those
that provide additional information in the log message.

app/celery/reporting_tasks.py

@@ -20,7 +20,6 @@ from app.models import EMAIL_TYPE, LETTER_TYPE, SMS_TYPE
 @notify_celery.task(name="create-nightly-billing")
 @cronitor("create-nightly-billing")
 def create_nightly_billing(day_start=None):
-    current_app.logger.info("create-nightly-billing task: started")
     # day_start is a datetime.date() object. e.g.
     # up to 4 days of data counting back from day_start is consolidated
     if day_start is None:
@@ -67,7 +66,6 @@ def create_nightly_billing_for_day(process_day):
 @notify_celery.task(name="create-nightly-notification-status")
 @cronitor("create-nightly-notification-status")
 def create_nightly_notification_status():
-    current_app.logger.info("create-nightly-notification-status task: started")
     yesterday = convert_utc_to_bst(datetime.utcnow()).date() - timedelta(days=1)
 
     # email and sms