Remove deprecated `AsServiceBusMessage` object extension

[stijnmoreels]

Is your feature request related to a problem? Please describe.
Previously, we used the object extension AsServiceBusMessage to create a ServiceBusMessage instance from a custom object but this was replaced by the ServiceBusMessageBuilder instead.

Describe the solution you'd like
Remove the AsServiceBusMessage to create a ServiceBusMessage instance in the Arcus.Messaging.ServiceBus.Core project.