Remove signal parameter from consumer

[rgraber]

The signal parameter of the consumer init method and the management command have been deprecated and should be removed. It may be necessary to make them optional first, release that, then remove them as a new major version.

A/C:

• Make optional in openedx-events (and event-bus-redis if it isn't already. it's already optional in event-bus-kafka)
• Remove parameter from all active consumers
  • make sure to test on stage before removing in prod
• Remove parameter from openedx-events
• Remove parameter from EBK (and let Axim know they can remove it from EBR)