Stop expecting letter contact block in service JSON

We’re removing it for performance reasons.

This means removing the old pages that edited the letter contact block
when it was stored directly on the service, rather than the current
model where a service can have multiple contact blocks.

app/models/service.py

@@ -36,7 +36,6 @@ class Service(JSONModel):
         'id',
         'inbound_api',
         'letter_branding',
-        'letter_contact_block',
         'message_limit',
         'name',
         'prefix_sms',