Remove redundant override for receipts worker

For some reason this doesn't work in practice: STATSD host is set
to the expected host when I run `cf env ...` in all environments.
Conversely, it is set to "None" for notify-api. I suspect this has
something to do with specifying the value twice [1][2].

It's unclear why this was disabled when the original work (renamed
from callbacks [3]) was added [4]. Enabling StatsD means we'll get
metrics for tasks executed by this worker, which we're missing.

(I suspect the config was copied from the app manifest [5], where it
makes sense, since the app can expose a /metrics endpoint instead.)

[1]: f3fdd3b09b/manifest.yml.j2 (L132)
[2]: f3fdd3b09b/manifest.yml.j2 (L153)
[3]: https://github.com/alphagov/notifications-api/pull/3206
[4]: 9d5b629dad
[5]: 39d7c347b0/manifest.yml.j2 (L7)

manifest.yml.j2

@@ -23,9 +23,6 @@
   'notify-api-sms-receipts': {
     'NOTIFY_APP_NAME': 'api',
     'disk_quota': '2G',
-    'additional_env_vars': {
-      'STATSD_HOST': None
-    },
     'routes': {
       'preview': ['api.notify.works/notifications/sms/mmg', 'api.notify.works/notifications/sms/firetext'],
       'staging': ['api.staging-notify.works/notifications/sms/mmg', 'api.staging-notify.works/notifications/sms/firetext'],