Remove the last mention of RabbitMQ (in start.py) (#5189)
Remove the last mention of RabbitMQ (in start.py) (#5189)

RabbitMQ was removed in favor of gRPC in 2016. While there have been
many commits to remove the AMQP-related code, this last mention in
start.py persisted.
jcjones/boulder,letsencrypt/boulder,jcjones/boulder,letsencrypt/boulder,bmw/boulder,jcjones/boulder,jcjones/boulder,bmw/boulder,letsencrypt/boulder,jsha/boulder,jsha/boulder,jsha/boulder,bmw/boulder