setup: Remove unused RABBITMQ_NODE.
setup: Remove unused RABBITMQ_NODE.

This reverts commit 889547ff5ed22205287245a8416d6bda43cd2d9c.  It is
unused in the Docker container, as the configurtaion of the `zulip`
user in the rabbitmq node is done via environment variables.  The
Zulip host in that context does not have `rabbitmqctl` installed, and
would have needed to know the Erlang cookie to be able to run these
commands.

kou/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,kou/zulip,andersk/zulip,kou/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,rht/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip