Inherit don’t duplicate API client constructor

This removes some code which is duplicative and obscure (ie it’s not
very clear why we do `"a" * 73` even though there is a Very Good Reason
for doing so).

app/notify_client/api_key_api_client.py

@@ -7,8 +7,6 @@ KEY_TYPE_TEST = 'test'
 
 
 class ApiKeyApiClient(NotifyAdminAPIClient):
-    def __init__(self):
-        super().__init__("a" * 73, "b")
 
     def get_api_keys(self, service_id):
         return self.get(url='/service/{}/api-keys'.format(service_id))