Get rate limit from service.rate_limit column (not config)

PR #1550 added the rate_limit column to the Service table.

This PR removes the rate limits from the config and uses rate_limit from
the Service model instead. Rate limits are still separated into 'team',
'normal' and 'test', but these values are the same for a service.

Pivotal story https://www.pivotaltracker.com/story/show/153992529

tests/app/service/test_rest.py

@@ -233,6 +233,7 @@ def test_create_service(client, sample_user):
     assert not json_resp['data']['research_mode']
     assert json_resp['data']['dvla_organisation'] == '001'
     assert json_resp['data']['sms_sender'] == current_app.config['FROM_NUMBER']
+    assert json_resp['data']['rate_limit'] == 3000
 
     service_db = Service.query.get(json_resp['data']['id'])
     assert service_db.name == 'created service'