[TASK] remove journey link from transactionalEmail

[JoernBerkefeld]

when "r__journey_key" is set on transactionalEmail, it means the journey was created based on the transactionalEmail.
Journey builder does it the other way round: create the journey first and then use the publish API to auto-create the transactionalEmail. in this scenario, however, there is no link from transactionalEmail to journey but only the other way round. instead, there is only an empty journey attribute:

 "journey": {},

mcdev should limit itself to this journey-builder scenario and disregard that the documentation only offers an alternative route.

that should also include removing the journey create option from the create callout!