Remove mentions of `backoff` from RabbitMQ component documentation

[renatolfc]

Describe the issue

dapr/dapr#1798 removed backoff support from the RabbitMQ pubsub component, but the docs still mention backoff support.

URL of the docs

https://docs.dapr.io/reference/components-reference/supported-pubsub/setup-rabbitmq/

Expected content

There shouldn't be any backoffs there.

[msfussell]

Completed