Remove unused client method

The app is now always getting an organisation by its `id` (or domain),
and never by `service_id`.

This means that the client method and associated mocking can be removed.

I think this must come from a time when the service response didn’t
include `organisation_id`, but now it always does.

app/notify_client/organisations_api_client.py

@@ -62,9 +62,6 @@ class OrganisationsClient(NotifyAdminAPIClient):
     def update_organisation_name(self, org_id, name):
         return self.update_organisation(org_id, name=name)
 
-    def get_service_organisation(self, service_id):
-        return self.get(url="/service/{}/organisation".format(service_id))
-
     @cache.delete('service-{service_id}')
     @cache.delete('live-service-and-organisation-counts')
     def update_service_organisation(self, service_id, org_id):

tests/conftest.py

@@ -3138,22 +3138,6 @@ def mock_get_no_organisation_by_domain(mocker):
     )
 
 
-@pytest.fixture(scope='function')
-def mock_get_service_organisation(
-    mocker,
-    agreement_signed=None,
-    organisation_type=None,
-):
-    def _get_service_organisation(service_id):
-        return organisation_json(
-            '7aa5d4e9-4385-4488-a489-07812ba13383',
-            agreement_signed=None,
-            organisation_type=None,
-        )
-
-    return mocker.patch('app.organisations_client.get_service_organisation', side_effect=_get_service_organisation)
-
-
 @pytest.fixture(scope='function')
 def mock_update_service_organisation(mocker):
     def _update_service_organisation(service_id, organisation_id):