Compatibility with Plugin API changes in rabbitmq-server:bug24926

In bug24926, we're giving users the ability to enable and disable plugins at runtime (without a restart), so plugins that make configuration changes need to provide a means to undo them.

In this patch I've only reversed the registration of the x-recent-history exchange in rabbit_registry. I have not written a "cleanup action" to remove the mnesia tables, but if you think that would make sense (when the plugin is disabled at runtime) then I will do so and update the PR.

Please note that this patch will remain meaningless until bug24926 is merged into default, although it won't hurt either (since the extra boot_step attribute will simply be ignored).