Remove update_organisation_name model method

It’s weird to have a method just for updating one attribute. I think
the reason for doing this was to only invalidate the
`organisation-{}-name` cache when absolutely necessary, but:
- we don’t need a separate method to check whether it’s the name being
  updated
- it was easy to get around this by calling
  `OrganisationsClient.update_organisation` directly, leaving a stale
  value in the cache

app/notify_client/organisations_api_client.py

@@ -56,11 +56,10 @@ class OrganisationsClient(NotifyAdminAPIClient):
         if cached_service_ids:
             redis_client.delete(*map('service-{}'.format, cached_service_ids))
 
-        return api_response
+        if 'name' in kwargs:
+            redis_client.delete(f'organisation-{org_id}-name')
 
-    @cache.delete('organisation-{org_id}-name')
-    def update_organisation_name(self, org_id, name):
-        return self.update_organisation(org_id, name=name)
+        return api_response
 
     @cache.delete('service-{service_id}')
     @cache.delete('live-service-and-organisation-counts')