tractor/newsfragments/230.removal.rst

Drop stream "shielding" support which was originally added to sidestep a cancelled call to .receive()

In the original api design a stream instance was returned directly from a call to Portal.run() and thus there was no "exit phase" to handle cancellations and errors which would trigger implicit closure. Now that we have said enter/exit semantics with Portal.open_stream_from() and Context.open_stream() we can drop this implicit (and arguably confusing) behavior.