Obsolete PurgeOnStartup

[andreasohlund]

Since it is very likely to lead to message loss we should not provide api's like this out of the box.

Instead we should guide our users to our scripting page and instruct them to perform the purging explicitly as necessary. This means we need to add a "Purge" section to pages like https://docs.particular.net/nservicebus/msmq/operations-scripting#create-queues