Remove require_bind machinery from Publisher

`Publisher.require_bind` defaults to `True`, which means every publish causes an attempted HTTP call to a hardcoded URL where the RabbitMQ management plugin's API is expected to be. This is pretty ridiculous in production and unlikely to be configured in tests. It adds unnecessary overhead to every message publish and because `routing_key_is_bound()` returns `True` if the HTTP call fails, it's not serving any useful purpose.