setup: Remove unused RABBITMQ_NODE.

This reverts commit 889547ff5e.  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.

scripts/setup/configure-rabbitmq

@@ -11,9 +11,6 @@ else
     rabbitmqctl=(sudo rabbitmqctl)
 fi
 
-if [ -n "$RABBITMQ_NODE" ]; then
-    rabbitmqctl+=(-n "$RABBITMQ_NODE")
-fi
 RABBITMQ_USERNAME=$("$(dirname "$0")/../get-django-setting" RABBITMQ_USERNAME)
 RABBITMQ_PASSWORD=$("$(dirname "$0")/../get-django-setting" RABBITMQ_PASSWORD)