Turn off statsd wrapper for synchronous statsd calls during POSTs

This commit turns off StatsD metrics for the following
- the `dao_create_notification` function
- the `user-agent` metric
- the response times and response codes per flask endpoint

This has been done with the purpose of not having the creation of text
messages or emails call out to StatsD during the request process. These
are the three current metrics that are currently called during the
processing of one of those requests and so have been removed from the
API.

The reason for removing the calls out to StatsD when processing a
request to send a notification is that we have seen two incidents
recently affected by DNS resolution for StatsD (either by a slow down in
resolution time or a failure to resolve). These POST requests are our
most critical code path and we don't want them to be affected by any
potential unforeseen trouble with StatsD DNS resolution.

We are not going to miss the removal of these metrics.
- the `dao_create_notification` metric is rarely/never looked at
- the `user-agent` metric is rarely/never looked at and can be got from
  our logs if we want it
- the response times and response codes per flask endpoint are already
  exposed using the gds metrics python library

I did not remove the statsd metrics from any other parts of the API
because
- As the POST notification endpoints are the main source of web traffic,
  we should have already removed most calls to StatsD which should
  greatly reduce the chance of their being any further issues with
  DNS resolution
- Some of the other metrics still provide value so no point deleting
  them if we don't need to
- The metrics on celery tasks will not affect any HTTP requests from
  users as they are async and also we do not currently have the
  infrastructure set up to replace them with a prometheus HTTP endpoint that
  can be scraped so this would require more work

app/dao/notifications_dao.py

@@ -77,7 +77,6 @@ def dao_get_last_date_template_was_used(template_id, service_id):
     return last_date
 
 
-@statsd(namespace="dao")
 @transactional
 def dao_create_notification(notification):
     if not notification.id: