Remove the 'old' Azure Service Bus message pump workings once the Arcus.BackgroundJobs package is updated

[stijnmoreels]

Is your feature request related to a problem? Please describe.
Since we rely in the Arcus.Messaging.ServiceBus.KeyRotation project on the Arcus.BackgroundJobs.* package (that uses again our messaging package), we have 'method not found's exceptions when we break the contract of the Azure Service Bus message pump.

Describe the solution you'd like
Once the Arcus.BackgroundJobs.* package is updated with our new messaging package, we can remove the 'old' workings of the message pump.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
PR with backwards compatible Azure Service Bus message pump #175