Remove all service postage references

Since service.postage field has been removed from our model
and database.

app/models/service.py

@@ -34,7 +34,6 @@ class Service():
         'name',
         'organisation_type',
         'permissions',
-        'postage',
         'prefix_sms',
         'research_mode',
         'service_callback_api',

tests/__init__.py

@@ -144,7 +144,6 @@ def service_json(
     organisation_type='central',
     prefix_sms=True,
     contact_link=None,
-    postage='second',
 ):
     if users is None:
         users = []
@@ -173,7 +172,6 @@ def service_json(
         'inbound_api': inbound_api,
         'service_callback_api': service_callback_api,
         'prefix_sms': prefix_sms,
-        'postage': postage,
         'contact_link': None,
     }