Remove clear_rabbit_queues script

The clear_rabbit_queues script has been around forever, but it doesn't
appear to be widely used - e.g. there is no user documentation for it
and I don't see any mentions of it on mailing lists or Q&A forums.

Also, RabbitMQ now includes a management plugin and CLI which allows
you do do e.g.

  $> rabbitmqadmin delete queue name=$topic
  $> rabbitmqadmin delete queue name=$topic.$host
  $> rabbitmqadmin delete exchange name=nova

oslo.messaging does not expose an API equivalent of this command since
even the existance of a queue is transport driver specific. We could
consider adding transport-specific admin CLIs, but probably not where
the transport already has suitable admin CLIs elsewhere.

For reference, this script was added by this merge:

  https://code.launchpad.net/~mdragon/nova/lp794635/+merge/71437

blueprint: oslo.messaging
Change-Id: If5e6354e8ace4c0b4425c96de558a5cbab4c98bd
Merge "Remove clear_rabbit_queues script"