Remove the `CloudEventBackgroundJob` message pump implementation

[stijnmoreels]

Is your feature request related to a problem? Please describe.
When refactoring the message processing from the message pump to the message routing, the message pump is having less responsibility and is now only used for customization and not regular message processing. The consequence is that the CloudEventBackgroundJob message pump is made obsolete and replaced with CloudEventMessageRouter.

Describe the solution you'd like
Remove the CloudEventBackgroundJob in the Arcus.BackgroundJobs.CloudEvents project.